@MikeRN 

At this time TP-Link does not sell PLC adapters outside of kits.  So you would have to puchase mulptle kits to achieve what you are looking at doing.  To add Kits into an existing setup just take the Powerline password printed on the label on the back of the new adapter.  log into the interface of the existing adapter and click on add and input the code.  Once added simply plug it in and enjoy.