I do not agree that DROs are so expensive, entry level units cost as much as only a few ruined parts?A good set of DROs will cost close to what it costs to convert to CNC.
I hope to, some day, be able to make something like that... very cool indeed!! And manually done... wow...With a little ingenuity and enough time (remember it's a hobby!) it is possible to do anything on a manual mill.
You are quite right, turning the motor generates electricity as I discovered when I pushed the table on my 3D printer back quickly. That caused the display screen to light up. No damage was discovered but I'm a little more careful at how fast I push the table back now. Turning it slowly keeps the voltage produced quite low. One would not normally turn it fast enough to do damage by hand.One concern I have had with turning a stepper motor by hand (de-energized, as Don notes), is that, as I understand it, turning the motor generates an electrical pulse. Somewhere or other I have read cautions about smoking the electronics this way on a 3d printer - don't know if that would apply to a more robust setup such as pictured above.