[Exim]Virtual Domains

I know that this is probably a RTFM question, but I can't seem to locate it.

Here's the situation:

I have a machine set up that processes mail as user@???, and
when a message goes out, the From: line contains host.domain.net as the
sending host (obviously...)

I am wanting to set up another domain, domain.org, and point the MX record
to an IP address of host.domain.net (using IP aliasing, so the two sites
won't get confused...), and I read in the docs how to set Exim to accept
mail addressed to domain.org, but I want to know how to tell Exim to append
"@domain.org" to the end of certain local parts, but "@???" to
the rest.

Is this possible?

TIA,

--Greg