You would need to add an encoder to the DC motor. Then modify the Marlin firmware to read the encoder. Or insert an extra component that takes the step+direction signal and counts the divisions on the encoder. You need an H-bridge driver for the DC current.
You would also need more gearbox as a DC motor doesn't perform well at 10RPM whereas a stepper is perfect for that speed.
Why not just get a bigger more powerful stepper? There are many different power ratings available in the NEMA17 case size and going up one case size will be smaller than the DC motor and gearbox.
RAMPS has all the stepper drivers on the big PCB. I haven't looked but I expect that it will be easy enough to hijack the step+direction and take that off-board to a new driver for your bigger stepper motor.