can an interior motion sensor be installed to an existing flourescent light fixture?
So you're looking for something like this?

It's an "Occupancy Sensor Wall Switch", which combines the functionality of an on/off pushbutton with a PIR motion sensor.
You would simply remove your existing switch and replace it with something like one of these.
When it senses movement it turns the lights on, and then after a pre-determined period of no activity it automatically turns them back off again.
You can also use the pushbutton to manually toggle the lights.