If I connect a rechargable 9v battery to the screw terminal of the asset tracker’s GPS shield - can the shield charge the battery when I connect a USB cable ?
No. The asset tracker shield does not have charging functionality; the Electron does. The Electron can only charge a lipo battery connected to it’s battery terminals (not the external power terminal on the asset tracker).
You’d need to find a 9v battery charger and put it in the circuit somewhere (where would depend on the charger probably).