If this is wanted for an "oh crap" my shift motor died kind of situation, I supposed one could fabricate a spacer that allows room for a manual handle to fit in between. The catch is what is the angle of sweep the handle has to make to make each selection? We have valves on the aircraft I work on that have handles/indicators to do that but the sweep from open to close is only about 30 degrees. So, the sweep fits inside where the screws mount the motor to the valve.
The other thing to think about, if handle sweep allows the modification to be possible, is will having an opening like that open the mechanism to mud, water, and dirt contamination that could cause other problems? I've never had the motor off. So, I don't know how sealed the motor and the actuator input is on the transfer case. One issue we have with the before mentioned valves is water getting in between the motor and the valve that freezes at altitude causing valve actuation to fail. The fix is to fill the gap with silicone grease but one trip to a hot location, like the Middle East, causes the grease to melt and run out of the gap. A ground vehicle will have much more of a risk than an aircraft to stuff getting in that gap.