[exim] Looking up catch-all from a file

Hello,

I want to configure a catchall router in exim, which does the following:
if /etc/vmail/$domain/catchall exists, redirect to the username
contained in that file.
if not, no catchall!

How do I check in exim if a file exists?
How do I read out the whole content of a file as the redirect username?

Thanks!
Nathan