It sounds like you want that particular phone to be able to go unseen when using the phone on whoever has the extension programmed as a button. It can't work that way. If it is assigned on a phone, it will show when it is in use no matter what. You can assign as many extensions as you want to a phone as long as you have buttons that you can assign them to. But i think you are trying to assign an extension to an extension, for instance, if my phone is ext. 222 but i want to use my same phone as ext. 222 and add,say 223, and be able to switch between the two when using the phone, the answer is no.
Gabriel