Do you wish to forward all mail to a specific address to somewhere else, all mail sent from a specific address, or a combination?
The first you can do in the Mail and FTP Users setup screens, by setting a forwarding address for the destination users. For the second, you can use the “Canocial Mapping” settings in Webmin’s Postfix module. For the third, you’d need Procmail rules for the destination users (you can create them in Usermin).