NAS-Port and IP pool

Hi, I'm hoping someone can assist me with my problem with assigning ip addresses with freeradius 1.1.6 on OpenBSD 4.1. I'm using freeradius to with an Ericsson CGSN and now I'm trying to assign ip addresses to my Mobile Stations. The problem is that the CGSN makes requests to freeradius always using the same IP and always port 1, which means only one IP is assigned. After a lot of searching I've come to the conclusion that freeradius uses NAS-port and NAS-ip to uniquely identify the clients. Is my conclusion correct? How can I assign IP addresses to my MS using another unique id, e.g. IMSI or MSISDN - which I can include in the request? /Danial