I made a dovecot master user plugin. If it doesn't work, just let me know and i'll look at it. I havent used it in a while. It may actually not work with 2 '@'s, now that I think about it. Anyways, just let me know.
Get it here: http://roundcube-plugins.googlecode.com/files/dovecot_impersonate-1.0.tgz
Cor
List info: http://lists.roundcube.net/dev/ BT/aba52c80