Authenticating with CallingStationId only

Hello, We have set up a FreeRADIUS for authenticating GPRS-users. However, we would like to authenticate based on CallingStationId only, i.e. not ask users to enter either user-name or password on a Mobile Station level. We are connected to a NAS sending us "void/void" if user/pw is not entered, and of course, the NAS sends the CallingStationId. For a number of practical reasons, we would like to use the CallingStationId when authenticating. My questions are: is this possible at all, i.e. not entering user name and password and only use CallingStationId? A few hints on where to modify, in order to avoid having the request rejected due to no user-name as is the case now. Thanks in advance, Patrik Backentoft