You can get RFID tags that have a constant value. But I have never heard of a reader with an ID system. To solve this problem, what we can do is that we buy many RFID tags.. and all have a unique ID that is in the hardware and cannot be changed. You then have to bind a microprocessor at every reciever and need to program an ID into it using software. Mind it this ID may be subject to change on burining a new program with a different hardcoded ID. You can then easily do the remaining part using XBEE by creating a network.
Utkarsh Mathur