Can a dusk-to-dawn fixture be converted to a motion fixture?

No can not convert photo eye to motion. To add a motion detector you would still need the factory installed photo eye so the light would only light at night. If you want a motion feature simply add a motion detector to the supply line of the light and mount detector on your pole or building below the light fixture.

I do not believe so as the current dusk to dawn sensor is on top of the fixture and has a small profile. Unless the manufacturer makes a replacement specifically designed to replace the sensor, then no I do not believe you could convert this fixture. However, this is an excellent light and has great coverage, if you do not have to have the motion sensor, you will still be very happy with this light.