NAS-port higher than base IP address

Hello, I have a question regarding the feature to assign the Framed-IP-Address with the plus ("+") sign to add a base address to the NAS port number. I have a Cisco router with analog dialin ports that come in on NAS-Ports numbered higher than the last part of the IP address that we want to assign. For example, NAS-Port 70 might be assigned for IP address xxx.xxx.xxx.65 regardless which of several users logs in on that port. Is there a way to make this assignment in the users config file? Thanks. ---Hillel