I can think of two ways off the top of my head.
Use a domain alias in LDAP, see the docs for how to do this, very easy.
Another alternative is to simply add a rewrite rule in the configuration. The downside to this is that because it is not contained in LDAP should you ever create a 2nd server, you'd have to do it there as well. I deal with large sites all the time so I try and do as much as possible in LDAP. Helps when you have say 12 MTA only machines. ;-)
Right now you probably have something along the lines of
domain.com$U%$D@machine.domain.com
To do what you asked, make domain and domain2 identical add the following
domain2.com$U%domain.com
Here's something you might helpful.
cd msg-<instance>
cp imta/config/imta.cnf test.cnf
vi test.cnf (make your changes)
./imsimta test -rewrite -config=test.cnf user@domain2.com
If you run that with say Greg@Domain.com and Greg@Domain2.com you should get the same results.
-Chad-