Hi wgmltd,
Assuming your VFD accepts an Analog signal you might read the KStep Analog Output section here:
Using KStep
There is an example C Program to output speed to KSteps Analog output. See:
\C Programs\KStep\KMotionCNC\Spindle_S_Kstep.c
General Spindle control info can be found here:
KMotionCNC Spindle Control
HTH
Regards