If they want to standardize on username@ourchurch.com:

1. Get a fixed IP.
2. Get a domain name.
3. Run a sendmail variant (strongly do not recommend sendmail itself: qmail, exim, etc. are better).
4. Let 'em use whatever client they want.

If they still want AOL, etc to be primary accounts:

In my experience, it's best to let the client software do multiple account merging, not the server. You're basically asking the server to pretend to be a client, which rarely works well--might as well have each client do it in its own finicky way. Most MTAs (email servers) want to deal with SMTP communications, not POP3 or IMAP.