[Dovecot] setting up both system password users AND virtual users

[Jim Pazarena]

I'm afraid that I am a rookie at dovecot, and cannot seem to figure the configuration
for a dual system & virtual setup. For now I am using POP3 ---only---

I've got the "system" working, but cannot figure out how to add the virtual system.
The docs aren't quite clear enough for me to understand.

The wiki example for dynamic passwd-file locations
shows:  mail_location = maildir:/home/%d/%n/Maildir
userdb passwd-file  etc etc

I am using mbox, not maildir, AND I've already used mail_location for my system accounts
with the following line:
mail_location = mbox:~/mail/:INBOX=/mail/%u

In the case of my virtual users, the mail is stored in INBOX=/mail/VIRTUAL/%d/%u
and the password file is stored in /exim/etc/VIRTUAL/%d/passwd

would someone kindly help me with this?
thanks!