FreeRADIUS authfile equivalent

[Steve Forman]

Greetings,

	I'm attempting to migrate a RADIUS service from Interlink RADIUS
to FreeRADIUS and am running into a bit of a problem I was hoping you
could help out with.  Basically, I have several different domains which
would all be separated by an @ sign plus the name of the domain.  What
I'd like to do is use separate local "users" files for each of these
domains.   In Interlink RADIUS, I simply use the "authfile" as the "glue"
to make this work.  e.g.

domain1.com	FILE	users.domain1.com
domain2.com	FILE	users.domain2.com
domain3.com	FILE	users.domain3.com

Each "users.domain.com" file would then contain the username and
password for every user in that domain such that  if
joesmith at domain1.com logs in, he will be authenticated with the
"joesmith" password contained in "users.domain1.com".  However, there is
no authfile in FreeRADIUS so the question is:
How do I accomplish this same task with FreeRADIUS?  Any information
would be greatly appreciated.  Thanks in advance.