Nice devices 
From what I can see, it may be possible to hack a cheap Zigbee contact sensor to use as transmitter. These sensors use a reed (magnetic) switch, which you can desolder and replace by two wires that you can subsequently use with the N.O. contacts of your PIR sensors.
Similar to the “rain meter hack”: Portable rain drop sensor made with Xiaomi door sensor
The Aqara sensors last a long time with a battery, although using them outside will limit their battery life if it tends to get cold.