I have an ceiling light fixture in the hallway entrance to my co-op apartment. The swithch that controls this fixture is actually inside my apartment. It would be great if this light came on automatically when a person enters the hallway as this would make it easier to put a key into the lock of my apartment door. Can a motion sensor be installed on the fixture and how would it be wired or do I have to purchase a new fixtiure with the motion sensor installed on the fixture?

The ones I've seen have the motion sensor built into the fixture. They don't cost very much. You set the amount of time it stays on after activation on the fixture itself.