[Exim] Using the RBL selectively

This is my first post to this list, and I should forewarn you that I am by
no means an Exim expert. Please excuse my ignorance.

I have been using Exim for some time now with no problems whatsoever. Now I
would like to expand our usage of it to allow filtering based on the RBL.
The documentation seems to be fairly straight-forward, but I have one point
of uncertainty I'm hoping to receive guidance on. I would like to begin
using the RBL selectively with a handful of users at first and then later
include all users at their request. It looks like I can do this by
specifying all users with the recipients_reject_except tag. There are
roughly 100 users that I would want to do this for. Is there a way to point
this tag to a file, or can I just include a list separated by commas? Is
there a better way to do this? Any advice would be appreciated.

Regards,

Chad Butler