They are just encoders, they will not allow Mach to close the loop as Mach is Open Loop. Nothing can be done to change that.
There is a board you can buy from Ron Rogers that accepts encoder inputs and it will fault Mach if the encoder values are different from where Mach thinks the axis is. It however is not closed loop as it will not correct on the fly, it will just fault and allow you to restart and update from encoder values if you wish. Whether that will save a part depends on the error value you have set and the tolerance of the part you are machining, having a well set up stepper system should never need that.
Servos are closed loop from drive to motor, Mach tells the drive to move to a position and the servo drive will read the encoder and move the axis whilst watching the encoder to make sure it gets there at the correct velocity.
Hood