Postfix 2.0 + DBMail + SMTP AUTH

Hi all!

I'm looking to migrate the dodgy setup from hell to DBMail. Here's the 
problem:

* The current setup is:

Postfix 1.x + UW-IMAP (ugh...) + SASL + PWCheck

I want to move this across to:

Postfix 2.x + DBMail

Migrating existing mail isn't a problem, but migrating existing 
usernames and passwords is. All users currently have UNIX system accounts.

The main problems are:

1. Obviously, all passwords are encrypted. It seems that getting this to 
work with SMTP AUTH can be problematic
2. All mail accounts have usernames that are not the same as their mail 
addresses (for instance, lisa@xxxxxxxxxxxxxx has the UNIX username lisasd).

Does anyone have any insights, advice or pointers to offer?

Best regards,

Chris