I have a light with a motion detector on-board. Would it be possible to add another detector, away from the light fixture, and still keep the one that is on the light functional?

This may be a possibility:

Add-On Motion Sensor for Motion-Activated Security Light, Motion-Activated Lighting Control - HS2144 - Smarthome (http://www.smarthome.com/25507MS/Add-On-Motion-Sensor-for-Motion-Activated-Security-Light-Motion-Activated-Lighting-Control-HS2144/p.aspx)