RE: does this exist (making sozbot)
You can order just about any current TX on ground frequency. That's not a problem.
Getting the main gimbals to just operate 1 channel each instead of 2... might be a problem. You can simply not plug anything into the channels that moving the sticks to the side would operate. I don't believe the 2 channel 2 stick transmitters would allow you to rotate the steering stick for your purpose. (and then you'd be short channels from your requirement...)
You CAN dismount the potentiometers that are operated by the side movement of the sticks, and drll the case to mount them as dial operated channels... And its possible to add "detents" for making it easier to repeatedly hit the same position on those dial operated channels. You would need to purchase (or scavenge from a bad TX) other potentiometers to put in the gimbals to hold them together. (doing it this way... no soldering)
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There have been lots of articles on making a servo so it can operate as a geared drive motor. You have to be sure to disengage the centering pot from the output gear or yuo will have done nothing by removing the travel stops. The stops are only there to protect the centering pot from overtravel. If the servo hits the stops... it generally means the centering pot has failed anyway... (so the stops are kind of pointless[&:])