I need you to edit Marlin firmware to make it support 6 stepper motors as 6 axis (X, Y, Z, A, B, C) and 6 endstops for that axis.
Now, Marlin supports 3 axis (X, Y, Z) and 3 extruders (E0, E1, E2).
We have Megatronics 3.0 board which has 6 stepper drivers and 6 endstops.
Now, E0, E1 and E2 can be controlled, but not as main axis, they don't have endstops and theirs options like feed rate (and others) cannot be set up separately.
The best would be if you would set up git repository, fork official Marlin firmware there and make changes which we can test and approve.
I'm here if you need more information.