Porting from TMobile prepaid to AT&T and getting added to an existing family plan

Hi, 

I'm looking to port my prepaid TMobile number to AT&T prepaid. I want to get added to an existing family plan.

I went to the store today and was told that it's not possible to get added to an existing family plan with ported number. I am not sure if this is entirely correct. Please let me know if it is at all possible.