I know very little about qmail and am not a huge fan of it. I'm confident there is a way to do what you want, but you may have to hunt around a bit to find it.
Qmail is made up of several pieces and it's possible to modify just one piece and leave the rest intact. You'll probably want to find a patch to the 'qmail-remote' piece since I think that handles SMTP connections.
Exim is another MTA (mail transfer agent), like sendmail, qmail or postfix. I believe Cpanel uses exim as it's MTA. I've been using exim for many years and it's the one I'm most comfortable with.