Quirky question about rewriting usernames

Hello all. Here is my issue. This is very weird and would only affect one NAS. I'm not sure freeradius is capable of this. I want a username that comes in to check for an @domainname. If the domainname is there I want it to be stripped and added back later. If the domainname is not there I'd like it to continue and have to domainname added later in the authentication process. I hope this makes sense and any help is appreciated Cliff