[exim] global .forward file

Is there any way to configure a global .forward file?

I'm playing around with vexim <http://silverwraith.com/vexim/>, which
adds virtual users to exim. Since these virtual users don't really
exist on the server, there is no place to add a .forward file.

What I really need is a simple filter that does this:

if $h_X-DSPAM-Result: is "Spam" then save mail/Junk

for all virtual users. The only other complication is that the above is
for mbox files; I want to use maildir, but I think the idea is the
same. I need to be able to put any suspected spam into the spambox, for
all users, without a .forward file for each user.

Thanks,

--Yan