Can I add an occupancy sensor to a 3-way circuit?
DMoore is spot on about using the 3-way occupancy sensor, but off a bit about needing to replace both switches with occupancy sensors.  The occupancy sensor installation instructions should show how to connect the device to a 3-way circuit, without replacing the other switch. 

Here is an example wiring diagram from the Lutron Maestro Occupancy Sensing Switch installation guide.



However, occupancy sensors only work if they can see you. So if the switch locations are not within line of sight, you may have to replace both switches depending on your operating preferences.