Freeradius with LAC setup

Dear all, im using the freeradius as proxy server with checking for the calling station id using the radcheck; i have a REDBACK used as LNS; and now want to use it as LAC too for a special context; i want domain (abc.com) to be proxyed and domain (xyz.com) to send back the needed attribute to the LAC without authenticate him on the freeradius can do this setup, and how? Thanks