[Exim] Using system-passwords for authentication

Here is an example of one of my current Authenticators:

fixed_plain:
    driver = plaintext
    public_name = PLAIN
    server_condition = ${if
crypteq{$3}{${extract{1}{:}{${lookup{$2}lsearch{/usr/local/etc/exim/relayers}
{$value}}}}}{no}
    server_set_id = $2

I am using a flat file to store the passwords for SMTP authentication.
I'm setting up SMTP encryption, and I want to start using
system-passwords to authenticate mail users. How would I modify the
above Authenticator to do a system-password check? I'm using FreeBSD
4.6.1, by the way.

Thanks!

--
Mark Edwards
Engineer
Mr. Toad's
San Francisco, CA