You can't. Non-shell user mail directories do exist, but I believe they're pretty much unreachable. They'd need to be a shell user to have a reachable mail directory.
That said, if they're using IMAP, then they should have a local copy of their mail. If they're a POP user, they should be popping the mail off the server anyhow and not just letting it sit in the inbox. POP also somewhat makes a local copy of their mail.
If you really want to keep your own backups of user mail, I see two options:
1) Convert users to Shell users and back up their Maildir
2) Write a script that grabs mail off the POP/IMAP server and files it away.