What, exactly, is the problem? How do you want Dovecot and XMail to work together?

I am setting up Dovecot to read from standard Maildirs in user home directories, because I am using a separate delivery agent (TMDA but would work for maildrop, Dovecot's deliver program and others) to deliver mails using mailproc.tab. So, even while XMail runs as root, Dovecot need not and there is no problem getting it to work just like with other MTAs. XMail can't help with delivery to Maildir because it runs as root, and unless you mess about with packet filters to non-root it you'll still end up using for instance XACLs to allow Dovecot to read the mails.

For authentication of XMail to Dovecot you would have to write external authentication helpers that use Dovecot's protocol. Or, you can use checkpassword or PAM-POP3 or something else to try getting Dovecot to authenticate against the active XMail list of users. Or you can just do what I do, maintain two databases and keep them in sync.

Cheers, Sabahattin