[Exim] SMTP Auth question

Hello,

I have SMTP auth working with exim (3.35, debian stable), and it works
fine. It uses a file with usernames and passwords generated by htpasswd
at the moment, and is in the form:

test@???:DhPbNVbjvCZBY

I am also using vm-pop3d to serve virtual POP accounts, and this uses a
htpasswd generated passwd file in the format:

test:DhPbNVbjvCZBY

but in the /mail/testdomain.com/passwd file, rather than a centralised
passwd file as with the exim passwd file. What I would like to do if
possible is to make both servers use the same password file in
/mail/<domainname>/passwd. Is it possible to get exim to take a
username, test@???, and then look the password for the local
part up in the /mail/testdomain.com/passwd file (ie. user 'test')?

Many thanks for any suggestions.

Will.