I think it should be possible to connect the DJI FPV remote controller directly via USB. If it shows up as a USB HID controller in your operating system chances are Freerider will be able to read the input of the controller and calibrate it correctly.
edit: (2021-03-26)
The internal input system has been completely reworked in the latest update.
You can try that to see if it works better with your controller.
During calibration，i pull all sticks in center position.As shown in the picture.These the remote- controlled circles are not in right place ,and the circles dont change linearly. They have step changes. I cant calibrate that properly.