@Pushtheriver Hello,

You could consider a AV600 which could work with your old powerline KIT  like model TL-WPA4220 but make sure they all work in the same powerline circuit or under same power meter. 

Add extra powerline unit into existing powerline network by pair button referring to the instruction:
https://www.tp-link.com/support/faq/1081/

Note:  press Pair button only for 1 sec, if the Power Led does not blink, press it again.
When the Powerline Led or House Leds are on, the pairing process is done.