Spindle control by SPI digital potentiometer

Hello everybody. I don´t know much about electronic systems so I am looking for some guidance. Me and some friends are converting a Sherline benchtop lathe to CNC. We plan to use a RPi with a CNC breakout board. So far we have been able to control the steppers but not the spindle. Originally, the spindle is controlled manually by a KBIC-240D DC motor speed controller. This board controls the spindle by means of a manual potentiometer and a lever switch. We are replacing the manual potentiometer with a digital one, especifically a MCP4131. However, this
potentiometer is controlled only by SPI communication protocol and as I understand it the default HAL file only sends PWM signal through the designated pin. 

How can I configure the HAL file so that the RPi can establish SPI communication with the potentiometer?

One thing to be very aware of is that the potentiometer on the KBIC controllers is at a very high voltage. It is referenced to DC+ so has 100V or so to earth.
If your digital potentiometer can't handle this then you might have problems.

I used this circuit to run a KBIC using PWM, which might be a better plan. Any logic-output (push-pull) optocoupler will work, but you probably need optical isolation to avoid letting the magic smoke out (DAMHIK)

photos.app.goo.gl/BpBKZob1TomX1gtSA

(It's shown with a VFD but it's basically a PWM-controlled potentiometer emulator)