As Solomon said, this is basically as arduous a task as writing a new software stack. Also, the cRIO is not very good compared to other processor options.
If your goal is to write code that is compatible with the RoboRIO and FRC driver station, you’re basically going to need to use a RoboRIO. If you don’t care about that as much, there’s the CTRE Hero and the VMX-pi shield, both of which have CAN bus support. The Hero probably has better out-of-the-box support for TalonSRXs over CAN though. If you’re willing to drive things through PWM instead of CAN almost anything could control your robot, including an Arduino + RC.
A cheap(er than a RoboRIO) yet still batteries-included package might be Vex RC electronics/software stack, which 254 uses for their T-shirt cannon.
The RIO is here to stay through at least 2021, so it might make sense to purchase more control system components, if you’re serious about testing a “real” FRC robot.