Not with the Unity product. It utilizes one of those two as its message repository. To be honest, if you are going to through the pain of installing that product, you should just integrate it from day one and be done. If for some reason you don't want to give the users single-inbox unified messaging you can create new accounts/mailboxes in a different message store.
Also, if you do not want to do unified messaging, you could also install Exchange on the Unity server. This was the old voicemail-only deployment model.
