No, they automatically act as repeaters once they are added to your network, so you’re good there. However, being added to the net work still doesn’t mean any specific nearby device will necessarily use them.
Updating the Network Tables
First, your old devices other than the hub won’t know that the new device exists until the Z wave network tables are updated.
With Z wave plus devices this will happen eventually through explorer frames, but it can take a week or more.
With older generation Z wave devices or if you want this to happen sooner, you have to run the Z wave repair utility. Once you do that, the other devices will know that a new repeater is now available.
Will the device use the New repeater once it knows it’s available?
Even once your sensor knows the outlet is available, it might not use it as a repeater. Message routes are selected in real time based on a number of factors, including message strength and other message traffic. It won’t necessarily route the way you as a human without all that information would expect it to. But that’s not anything to worry about. The algorithm is quite efficient, so unless you are seeing persistent problems, don’t worry about a particular route which seems unusual.
what the path report shows
It’s also really important to understand that the path report that smartthings displays in the IDE is not a typical zwave hub routing table. All it shows you is the single route most recently reported from the hub to the cloud, and those reports can be delayed by as much as 24 hours. And it might be a route that was just chosen for a particular message because the network was busy at that time, it’s not the most common route or the optimum route for that particular device. It’s just a snapshot of a route that a particular message took.
So while there is some information there, it’s not really useful for troubleshooting in real time.
Repeaters have the same range as nonrepeaters
If you put the outlet really close to the sensor that was having an issue, the outlet may run into exactly the same range issue. So it may not actually help much.
Normally when we are trying to improve a zwave range issue we locate the new repeater about 30 feet from the device that was having trouble. That’s the way that we build the mesh out. We’re giving the device it was having trouble a new route which should be quite easy to reach (30 feet is about half of Z wave plus range in a typical American residential home) But that is still getting the signal considerably closer to the next available repeater or to the hub itself.
If you put the repeater within 6 to 8 feet of the device originally having trouble, you may not actually improve message reach in any significant way. You just end up with two devices that have trouble getting back to the mesh. 
You may have read something about “beaming” and they need to place a repeater that supports beaming within about 6 feet of a Z wave lock. That’s a different kind of issue and hast to deal with how messages are handled for battery operated box. But it doesn’t apply to any other device class like sensors. You may have read something about “beaming” and they need to place a repeater that supports beaming within about 6 feet of a Z wave lock. That’s a different kind of issue and hast to deal with how messages are handled for battery operated box. But it doesn’t apply to any other device class like sensors. So that’s a special case and has to do with extra encryption that is used for locks.
The other special case is when there is a local architectural barrier to signal, like a sensor inside a metal outbuilding. In that case, we may try to position a repeater very close to a window or as a plug in the wall where we know that signal will be able to leak through to the other side of the wall. Or If the sensor was in the garage and the cars and other appliances were blocking the signal to it, so we used a Z wave light bulb as a repeater just to get signal up off the ground so it could pass over the metal objects. But again, that’s a special case where we are trying to deal with physical barriers to signal Transmission, not just extend the mesh so we can reach devices farther away.
So there are a few cases when we might be putting a repeater quite close to a battery powered device that had been having trouble reaching with a network. But most of the time if we just want to “strengthen the mesh,“ we will put a new repeater far enough away that it is giving us significant additional distance. For zigbee that would probably be about 15 feet. For Z wave plus about 30. 