Press Feature+4 on phone to find port number.
In programming, press LK4 then dial 10.
Use * button to scroll backward to port (all the way left).
Enter port number found from phone.
Enter new extension number.
Be sure there is not another set with same number.
Don't know how big your system is, or how your numbering plan is set up, or if you have LCR activated, or if you use prime lines, but be careful of numbering conflicts so the system doesn't think you are dialing a toll free number.