Hopefully I've understood your question corectly... forgive me if I didn't.
In qmail it's easily done through entries in /var/qmail/alias
let's say you set up email for user1 on the system. He/she will get the email address user1@yourdomain. If you want user1 to get email addressed user2@yourdomain you set up an alias, a file name .qmail-user2 which contains "&user1" without the quotes.