Where does the incoming call normally route? Is it a DID? Are you routing based on on caller ID, or all calls on a trunk?
You can set up a dummy extension, as a destination, and use that to forward outside. Keep in mind that the caller ID being sent (originators?) can sometimes be an issue depending on what your provider allows, and how you have things set up.
Perhaps you could provide a bit more information about what you are trying to accomplish.