There may be a specific term that describes this functionality but I have never worked with it before. A client is asking to have the ability from one main phone to be able to 'act' as another extension. They want to be able to choose an extension and then dial out from that extension. Specifically they want to be able to initiate conference calls from the other extensions. Currently when they are trying to do so, the conference calls are being brought into the local extension from which the calls are made, of course.
What is this functionality if it exists? Or does this user just need to walk over to the other phone and dial away?
You didn't described the environment, so generic answer is possible only. It's up to PBX to implement the feature in question. So if your PBX support it then it is possible, you are out of luck otherwise.