[Dovecot] Username mapping

[nodata]

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 at 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