How to adjust the throttle of ground rover to support -1..1?

Hi all,
I want MAV_TYPE_GROUND_ROVER to support both forward and reverse.
Now PX4/Pixhawk2.4.8/QGC only supports scaling the throttle of the ground rover to 0…1.
How to adjust the throttle of ground rover to support -1…1?

Thanks & Best Regards!