[Exim] Email forwarding with virtual users

Hi guys,
How would I get exim to preform the same function as a .forward file, when
the mail accounts are virtual users, which do not have a homedir on the
system.

Every mail account has a maildir /opt/services/vmail/<domain>/<local_part>
Is there a way I can get exim to look there for the .forward file?

Alternativly I am happy with having a file eg. /etc/mailforward which
would map say: foo@??? -> bar@???

What is the best way to get this done?

Thanks,
Daniel