There are too many things to say about how to convert the MPCNC to 3D printing, so maybe you can check my old build’s thread first:
No need to read everything, but you should find the answers to most of your questions there
Basically you’ll have to add:
-an extruding system (motor, hotend, temp sensor, cooling fan for the cold side, etc).
-endstops, for homing on all axes
-fans for cooling the part
-a probe system to measure bed’s flatness. This is not entirely mandatory but it helps so much in getting a decent first layer that you really should consider it from the beginning
-a bed heating system. Not mandatory if you plan on printing small parts with PLA.
You’ll have to adapt the firmware a little bit but we’ll be there to help you.
One thing is that you can’t use the autosquare function in CNC mode, because you’ll need 4 axis for the printer.
Good luck and don’t hesitate to ask if you have any problem 