@WelshPerson Hello, 

It is possible to add a single powerline unit as extender into your existing powerline network, but powerline KIT are normally sold by KIT and it is recommended to check local store or online store.

In theory, all the TP-Link powerline devices on sale can work together. In the real network environment, it would be better if we use the powerline devices with same powerline data speed in the same network. We can also use powerline devices with different data speeds together. Here are some recommendations:

So you could consider a 7/9 series as the extender, like TL- WPA7510 /TL-WPA9610. If  you only found KIT, in a KIT, there is a wired powerline unit and a wireless one, both could work as extenders, but only model with " WPA“ （W means wireless ）could provide Wi-Fi, the wired unit could extend Internet by Ethernet cable.