[Exim] Mailboxes on Virtual Domains

I have been running exim on our server for probably 6 months now, and it has
been quite stable. We are now beginning to add some more virtual domains,
however, and I would like to know the best way to implement this. I have
read the section in the spec document, and it was a little confusing. My
biggest question is, can you create seperate user lists for the different
domains? So I can have bill@??? and bill@??? hosted on the
same server? If so, could anyone help me figure out how to go about this?
Also, can I avoid creating regular system accounts for each user I want to
add? (obviously I couldn't create two accounts called "bill", so this gets
back to my first problem). How have other people implemented this? Any
help would be -greatly- appriciated!

Thanks,
Ryan Hayle