You know if there is no function to add torque, maybe you could emulate it.
func add_torque(state, torque_axis):
var force_pos = torque_axis.cross(Vector3(0,1,0)).normalized()
var torque = torque_axis.length()
var force_dir = force_pos.cross(torque_axis).normalized() * torque
state is, well
state. The direction of
torque_axis is the axis your adding torque about. Its length defines the strength of the torque.
Note: I made this function on the top of my head. I haven't tested it yet so bare with me.