Mixing (closed loop) stepper drivers on MKS main board
As far as I know, that should work. You need to define the correctly used drivers in your configuration. For the closed loop drivers, the board (via a break out board) has to provide the pulses to the motor driver (on the back of the stepper). If you use such a closed loop driver you need to disable the stepper driver type in your Configuration.h file for those you plan to insert a closed loop driver. The firmware will assume the A4988 driver is being used:

Now you need to set high enough values for the MAXIMUM_STEPPER_RATE (150000):

and the MINIMUM_STEPPER_PULSE (3):

in Marlin/src/inc/Conditionals_adv.h under HAS_DRIVER(4988).