taking a fixture/box off of a 3 way switch...

Hi -
I live in a old 3 decker in boston that got turned into condos. The back stairs go up into a headhouse on the roof that gives us access to a very nice roof deck.

Going up the back stairs, there are lights on each landing, and a light switch (3 way I think??) - the switches turn all the lights on or off.

The problem is there is a light up in the headhouse....but there's no switch.

What happens is - we go up to the deck in the evening when its sunny (so we don't turn on the hall lights)...and usually stay out till its dusk....and when we get back to the headhouse, its dark....and we have to go down some tricky 1890 era winders to get to the light switch.

Had an electrian over to do some other work - based on the layout of the stairs/headhouse, running the wires up to the headhouse for a switch would a real pain - lot of work, lot of digging in the walls, etc.

What I'm thinking about now is putting in a motion sensitive light (they sell them at lowes/Home Depot) in. We don't need much light, and it only needs to be on when we come in at night....so it might be perfect.

Any idea how hard, if its even possible, to take the light fixture/box up in the headhouse "out" of the circuit connected to the light switches? So it would be always powered?

I know my grasp of electrical terms is weak....but hopefully its clear enough that someone can give me a pointer...