openLDAP branches

Hi, I have a openLDAP server with multiply branches: dc=domA ->a list of users dc=domB ->a list of users dc=domC ->a list of users now I want to AAA from freeradius using this syntax: userX@domA, userY@domB.... how to setup radius.conf? I sucessufully reach ldap server with single basedn = "dc=domA" ...but how to automatize it? Thanks in adavance. v.