Problem using mbox format

Hi!

I catch my mails with fetchmail and they are delivered to my maildir  
in maildir format but I nedd to use my exisiting mbox.

Config:

# Configuration created Wed Dec 29 17:30:24 2010 by fetchmailconf 1.55  
$Revision: 5330 $
set syslog
set postmaster "username"
set bouncemail
set no spambounce
set softbounce
set properties ""
poll xxx with proto POP3 auth password and options no dns
        user 'yyy' there with password 'zzz' is 'username' here  
options keep
mda '/usr/bin/procmail -d %s'          # execute procmail

This is my .procmail:

# ~/.procmailrc
PATH="/usr/bin:/usr/local/bin"
MAILDIR="/var/mail"
DEFAULT="/var/mail/"
LOGFILE="/var/mail/procmail.log"
SHELL="/bin/sh"
VERBOSE="on"

Any idea what I could change to fill my mbox?

Thanks, Sandra