Hello & Welcome to Just Answer,
You MAY be able to add handsets to these systems, but let me ask a few questions.
May I ask:
1) the Model Number of the MAIN BASE UNIT of each system? I ask because the PLNC number you sent appear to be for AC power adapters - which usually plug into the bottom or back of the MAIN BASE UNIT. Base units usually have a model number in the format of KX-TG xxxx - were xxxx is a 4-digit number.
2) Will these system be used in the same or different locations (such as different homes)?
I see it has been a few days and I just wanted to follow up to see if you have any more questions concerning this issue or if the issue was resolved.
I look forward to any further questions or results that you may have.
Thanks, ***** *****