Probably well beyind the scope of a simple forum response, if possible at all. It would require a complete change to everything email, including how DirectAdmin defines virtual users vs system users, and how email is maintained, found, counted, etc.
I'm prejudiced though; look for other posts on qmail spread through these forums.
The only part of qmail I like is Maildir (though I personally feel the Maildir directory structure is a bit archaic and could be seriously simplified), and Maildir is already implemented (along with Dovecot) in DirectAdmin.
Jeff