[SR-Users] Kamailio/Freeswitch auth user and UserID

Hi I am new to Kamailio and would like some guidance on linking up with Freeswitch. I have an exiting hosted Asterisk PBX that uses a combination of extension and pbx username (eg 1001-126252581276) and I want to move it over to Kamailio/Freeswitch without re-configuring the SIP phones. I have tried Freeswitch "number-alias" but it does not seem like a good fit due to things like BLF not working and the added overhead etc. Is there a way to configure Kamailio to authenticate a user based on the above username and then send an extension to Freeswitch as the UserID instead, thereby separating the Kamailio authentication from the Freeswitch UserID? As I don't yet have a full understanding of the Kamailio/Freeswitch interaction, I don't know whether this will cause other issues? Thanks in advance Petarr