Depending on the exact details and your personal aesthetics, this could be very easy. Or quite difficult. But there are several choices if you’re in the U.S., and one for the UK.
project parameters
Here are the things that matter besides budget. (The costs on the solutions I’m going to list vary from $25 total to several hundred, but some people are happy to pay a lot more for aesthetics, so that part is just an individual decision.)
A) is any level of lag acceptable? Could you use an LED under cabinet strip that comes on with motion instantly in combination with waiting 2 or 3 seconds for the overhead lights to come on?
B) will you use smart bulbs? Or only dumb bulbs and a smart switch?
C) must the auxiliary have full control? Or would it be OK for it to have settings like 50%, 75%, and 100%?
D) how quickly do you need the status update from the switch back to smartthings? this affects two different things.
First, will it drive you crazy if you open your phone and it says the light is off when you can see the light is actually on, when you know that that status will update presently?
Second, do you intend to run any other logic off the auxiliary switch besides turning the master-controlled lights on?
For example, some people have a smart switch send a message to the hub so that it’s not just the kitchen lights that come on, but maybe the hallway lights in the next room. Right now, it’s not important what the other thing might be, it’s just important how quickly the hub needs to know that a person physically tapped that switch.
Obviously, like aesthetics and budget come there’s no one right answer to these. Different people will have different priorities. But they will affect the possible solutions.
philips dimmer. $25. fast, easy, and limited
If you get smartbulbs, Philips Hue makes a $25 dimmer switch that is very similar to the pico remote. It comes with a wall plate, or you can use it as a handheld remote. Battery-operated. It can toggle or dim a single group of up to 10 smart bulbs that are also controlled by SmartThings. No lag.
The main deficiency right now is it doesn’t fully integrate with SmartThings, and it doesn’t send the status back to the hub when the switch is tapped. Which means SmartThings still thinks the status is whatever the last master command is until it gets around to asking the master if anything is changed which could take a few minutes.
These went on sale in late September, almost immediately sold out everywhere, and will be in wide release in the U.S. on November 1. Best Buy, Staples and similar stores should carry them, as well as Amazon and other online retailers. Pretty much anyone who carries the Phillips hue bulbs. (The UK wide release is scheduled for November 30, and it’s available in Germany now.)
This one is mine. It’s on my refrigerator so my service dog can use it. (I’m quadriparetic with limited hand function.) The wallplate for this model also has magnets on the back giving you more options for easy placement. 
2) $250 Cooper Aspire RF9500. nearly perfect, but expensive
Cooper makes a battery-operated switch called the anywhere switch for just this purpose. Cooper Aspire RF9500. Comes in several styles and colors. As long as the master is a smart switch, you can use dumb bulbs with this. The switch will talk to the SmartThings hub, and the Cooper line is one of the only ones that supports “instant status update” which means that communication is very quick. If you get the matching Cooper master, you will have full control with the dimmer. If you match it to a different switch, controls may be a little bit limited. Very well engineered, Cooper switches usually come with a five year warranty, although I haven’t checked this particular model lately. You will need some custom code to make it work, but there are community members using it.
The biggest downside is the cost. Depending where you buy it, Cooper switches can easily cost $200 each. Also Read the description carefully: the faceplate is usually sold separately.
3 SmartenIT 3 Toggle. fast, easy, dim limited to presets, looks different
SmartenIT has a $50 battery operated 3 toggle switch which is my personal favorite for many similar situations. Fully integrates with SmartThings, so you can use it to change modes, run a routine, or change a set of lights. But these are just toggle switches. Each of the three has an on and off. So you can use one for on/off, one button for 50%, and one for 75% or whatever, but it’s not full dimming control. And it’s not going to match your other switches.
4 “Earth to Lutron. Come in, Lutron.” Dual control systems, might have lots of lag.
It’s true that Lutron doesn’t integrate directly with SmartThings. One reason I’m not putting a photo in the section, because I don’t want people to jump on thinking Lutron devices work with SmartThings. They don’t.
But, both SmartThings and Lutron Caseta have IFTTT channels. (But the Lutron wall remote is not part of it.)
Which means if you have a Lutron Caseta light system in one room, it’s pretty easy for SmartThings to send a request to the lutron controller to change the level of the lights. Except that request has to go by Internet! (So out to the SmartThings things cloud, over to the IFTTT cloud, over to the Lutron cloud, back to the Lutron controller at your house, over to the Lutron switch.)
So it could take some time for the message to travel all the way around and the lights to actually change. And if you push the button on the Lutron remote, it could take a long time for SmartThings to know the lights had changed, or it might even never know it. And you probably have to set up virtual switches in your SmartThings system to represent the Lutron switches, which can get complicated.
You can improve some of this somewhat by adding yet another controller into the mix, but let’s not even go there right now. You can’t fix all of it.
These limitations make Lutron Caseta a very easy way to add control of window coverings, because usually nobody cares if it takes the curtains 30 seconds to start moving. But they care a lot if it’s a light.
So I mention this option, because it does exist, but it really doesn’t solve your particular problem for most people.
5 Tablet dashboard
Another option is to use an inexpensive tablet as a control center dashboard in that place. Then you could change all kinds of things, not just the dim levels. This is another one that might be combined with motion activated under cabinet light strips. Cost for the tablets vary a lot, although you can find android tablets for about $50 that will work fine. Or if you just want a couple of buttons on it, a $20 Wi-Fi phone. Aesthetics are a big issue for some people, but that works both ways. Some people love this idea. So this is one of those where some people want to do soon as they see it and some people are like “just not what I want.”
Smarttiles.click is A third-party dashboard app, developed by a member of the community here, which is free to try and then you donate if you like it. Fully customizable, works in any web browser. Very popular for this purpose. There’s even a forum topic just on different mounting solutions people are using for it.
6 and remember those motion activated options. They could be combined with one of the above, or just used on their own as safety light
Adds cost, but a lot of people love this approach.
7 and then there’s the one we use at my house: echo for voice control of the lights, including setting dim level. “Alexa, turn kitchen lights on to 80%.”
My favorite. fantastic integration between SmartThings and Echo and works from anywhere in the room. Echo costs $179 but can do many different things. We have one in our living room that controls lights in several rooms, but coverage depends on your exact floorplan.
So, a number of choices depending on your answer to the first set of project parameter questions.