Ed,
I'm pretty certain we attempted to do this years ago and Paul at vodavi said it couldn't be done. And the reason we were trying to do it was very similar to what you are trying to do they wanted it in the event of not getting into the office they could forward. If my memory serves me correct we put it that main line in its own Pool group to get around it. This particular places was "squared" so it didn't really affect anything else.
Gary