Buried in the FRC firmware is logic that receives the enable flag from the DS & expects constant coms from the DS. First doesn't publish source code for the firmware for reason.
So either you need to run non FRC firmware on the cRio, not a realistic possibility for most teams. (mine included.) Or put a something that mimics a driver station on the robot. Conceivably something like a PI with a hacked DS that is constantly enabled. Obviously you would keep safety in mind when doing this.