wholesale issue

Dear All, i want to setup a freeradius as a proxy radius for a wholesale, and want to limit the access by using the calling-station-id; so user1@isp1 can connect only from any calling-station-id that belong to isp1, (ex, 555111, 333222) and user2@isp2 can connect only from any calling-station-id that belong to isp2.... i have all the calling station id which belong to the ISPs, but i didnt have the username for every ISP, and want to use the realm instead of the username to do that. can i use the checkval to check for the calling-station-id and realm, or is there anyway to do that?