You may be able to cause it to trigger if you place two filament bulbs operated under-voltage at a dim glow in front of it, you may have to tune the positioning a bit to get it right. Then switch them on alternately hoping that they will illuminate the alternate parts of the sensor. The spacing of the bulbs should be related to the detection 'beam' angle which can be different in different directions and zones and obviously will vary by model and manufacturer as well. The alternating frequency will need to be made to match the expected signal it would get if a person was crossing the detection 'beams' and getting it to match walking speed.
EDIT:
A domestic PIR sensor is designed to detect moving IR sources. They generally have a two active area detectors that have to have the areas alternately illuminated to create an alternating differential (pyroelectrically generated voltage) so they can detect a warm body moving but not a stationary heater. The alternating signal is normally achieved with the multi-zone Fresnel lens (or reflector) found on this type of sensor. To spoof it you need to place two sources that illuminate the two zones unevenly and then alternate their signal to mimic the flickering pattern that a moving person would create. A single source may not cause the proper alternating signal to be generated and be filtered out as spurious environmental changes.