Convert from user@domain to domain\user

Hi all I am a newbie to freeradius and I have searched the internet and all the documentation I can find, however I cannot find a simple solution to the following problem. I want to have the freeradius server act as a proxy and receive radius requests in the form user@domain but then pass these requests to another server that needs to have the request in domain\user format. It would seem to be a simple format issue which you should be able to create an output template or something but I don't have a clue. Running freeradius v 1.188.2.4.2.12 Rob Graham