[Dovecot] Username mapping

Hello. We're using dovecot with mysql for authentication, with postfixadmin for the admin web interface.

We are moving users from passwd file usernames to email@domain style logins.

Is there a better way to allow both sets of usernames (old and new) to work with dovecot than by adding extra copies of rows to the mailboxes table and changing the username? i.e. does dovecot support some kind of username mapping or aliasing?

Thanks a lot