New use for extruder port
If you are planning to use the printer via GCode entirely, then I don't believe it would be too complicated. For example, Marlin treats the extruder as another axis and uses absolute values for it (if absolute mode is on, of course). Since you want to have two steppers, you would have to connect them in either series or parallel (usually the Z motors are wired in parallel on RAMPS type boards, but many people are going for series since it has some benefits, one of them being that the torque will be much more similar between the two motors). Of course lower the stepper driver current to the minimum before connecting, then adjust it to whatever value you need.
I don't think it will work too well if it has to keep the roll tight - in that case, you could be better off by using only one stepper and some kind of force to hold it tight... maybe a small DC motor at low currents?

You can modify your printer firmware and set your own pin assignments for everything, for example in Marlin you have the pins.h file where every pin from the Mega is mapped to different headers and drivers. I don't know if there is any LOM slicer or firmware available, but you could just use GRBL.