PLEASE NOTE, the following are the instructions as to how to accomplish what you have requested. I don't make any representation as to the legality of what you are trying to do, and as such, shall remain harmless. I cannot speak to whether or not following these steps violate licensing for Cisco or Microsoft.

Yes, assuming that your exchange / unity box is also dc for your voicemail domain, you would perform the following:

In Active Directory (on your voicemail box), create a contact (Action > New > Contact). On the first screen, input first and last name and click next. Click Modify (next to email), select your mail transport protocol (most likely smtp) and click next. Input the mail address and click apply, then okay. click next and then finish.

Now, locate the mailbox for which you want to forward messages. Right click and select properties. On the exchange general tab, click delivery options. Click Modify and type the name of the contact you created (above). Click okay. If you still want the messages to go to that subscribers mailbox, check the Deliver messages to both... option and click Okay.