Although what you describe is technically possible, I just would not do this... if someone is not willing to dial 9 (or whatever) it is just pure laziness or stubbornness and that much unwillingness to change will just keep you coming back trying to do oddball one-off fixes so "it acts just like my old phone"... there is a reason they don't have their old phone and now have a new phone, adapt or get out of the way for crying out loud, it's not a big deal.
If one of my sales guys came to me with this question, I would give them the same answer as when they ask if the 5000 can use 9 for outside access... NO
(Sorry if I am little abrupt, it was a long, rough day!)