BTW, if your wiring would allow for a GE add on switch, it should also allow for the Linear/GoControl add on switch, and that one does have its own zwave radio and can communicate directly with the smartthings hub. It doesn’t use traveler wires to communicate with its own master switch; it always uses a virtual three-way.
So again, depending on your wiring, you could use a GE master switch and a linear/go control add on switch, with the add on switch talking directly to the smartthings hub rather than to the master. That means the Linear/go control switch doesn’t have to be on the same circuit. But it is a mains-powered switch.
Or of course, you could use the linear/gocontrol control switch with its own master switch. If you want an exact styles match.
So I’m not sure whether that one helps you out at all, but it’s not considered a “button” the way the ones are in the previous link that I gave you, so I did just want to mention it in case it helps in this specific situation.
Linear add on switch:
And here’s the Linear master switch, just in case you want to see that one. Note that this cannot work with a GE add on switch, as the wiring is different. But It can work with other virtual three-way devices as long as everything talks to the SmartThings hub first.
The linear/go control line is very popular and is carried at a number of different retailers, so shop around as prices do vary from time to time. Definitely check Home Depot, Lowes, and Best Buy as well as Amazon.