[Dovecot] "Real" users AND Virtual users

[Muhamad A. Martoprawiro]

Dear all, 
How can I use dovecot for both "real" users and virtual users?

I could configure dovecot for real users, by using these lines
in /etc/dovecot.conf:
  default_mail_env = maildir:%h/Maildir
  auth default {
  passdb pam {
    }
  userdb passwd {
    }
  }

To accept virtual users, I used these lines: 
default_mail_env = maildir:/usr/local/virtual/%u/
auth default {
passdb sql {
        args = /etc/dovecot-mysql.conf
  }
userdb sql {
        args = /etc/dovecot-mysql.conf
  }
}

But, again, how to accept both kind of users?
I use Postfix as mail server, and use LDAP/PAM
for real users that have home-directory on the system.
My OS is Mandriva 2007, and I use dovecot that is 
bundled as an RPM package (dovecot 1.9 rc7) in the OS. 
The content of file /etc/pam.d/dovecot: 
  #%PAM-1.0
  auth       required     pam_stack.so service=system-auth
  account    required     pam_stack.so service=system-auth

Thanks.

Regards, 
Muhamad