Hello   @Weledele 

You could have more powerline unit added and pair with the original powerlike kit but kindly note that they need to work and pair under the same powerline. 

Please firstly confirm your house extension (a garage/ barn/shed behind house and etc.) and the main building is under the same electrical meter. If they are under the same power meter, powerline adapters should work in the house extension.
Considering powerline signal strength loses over distance and most house extension has a certain range from the main building, it is hard to guarantee the performance when powerline devices work in your house extension. It is recommended to borrow a powerline kit for a test before you purchase one if possible.

To add more powerline unit,please consider powerline units of the same powerline speed like AV1000 Wifi powerline starter kit,like TP-WPA7517Kit or TL-WPA7510kit. The wired unit can also work as a extender and extend wired network connection.  

How can I select the powerline device?

How to choose a TP-Link Powerline product