Username with different @domain does not work

I have the requirement that the user name (login name) of each e-mail address of the registered user must comply. Example: the chat server has the @, the logins of users;, etc.

My problem: Some Jabber clients like PSI can use only user name with the domain name of the server (in this example @ for registration.

Question: What can I do, the user logins - regardless of their default (Server) domain - eg assign an alias?

Best regards,