My Company is doing the same thing. I found a really good tutorial on msexchange.org. Basicly though you have to use ADSI Edit, which is unsupported by M$, and if you use it incorrectly you can screw up your Active Directory. You have to limit permissions to the address lists you create with ADSI Edit. The Tutorial on msexchange.org for Hosted Exchange gives really good intructions on how to do this.
Support given as is, YMMV, all risk is on you.