any domain with multi domains

Hello

Is it possible for a user to use any of the available domains with multi domains?
example
user.domain1.com
user.domain2.com

the reason i say this is cuz the username is already taken. might as well give the user the ability to use any of the available domains per his/her changing taste. Its ok if all non-default domains redirect to user's actual user.domain1.com address (even if someone types user.domain2.com).