Isolating a bulb from stairwell timer
I suggest that you don't have the right to interfere with communal wiring and lighting, one because it is not part of your property (leased, rented or owned) and two, because you may not have the qualifications or insurance coverage to do that work.

However, If you want a light for your door controlled by a motion detector then there are LED lights with batteries available to do what you need.