Asterisk / FreePBX and Exchange 2010 Unified Messaging

I have Asterisk connected to my Exchange 2010 server and now I would like to configured individual mailbox’s to use Exchange for voice mail while leaving a majority of the extensions using Asterisk for voice mail.

How can I configure individual extensions so that voice mail is directed to Exchange?

FreePBX 2.8.1.4
Asterisk 1.8.1.1

Thanks -greg