Hi jossa,

Yes JP33 Pins 5,6, and 7 are the KSTEP Analog Output pins.

Apply KFLOP's +5V to JP33 pin 5.
Apply KFLOP's GND to JP33 pin 7 and also Spindle CN17 GND
Then JP33 Pin 6 will be a 0-5V signal that you can connect to the Spindle CN17 "Input".

Configure KMotionCNC | Tool Setup | M3-M9 | S = Exec Prog Thread=2 Var=1 <>\C Programs\KSTEP\KMotionCNC\Spindle_S_KStep.c

Check with a Voltmeter (DC Volts) to JP33 Pin6. Commanding in KMotionCNC MDI S100 should read about 1.0V, S250 should read about 2.5V, S400 should read about 4.0V. etc.

Yes there are two "Relay" driver outputs on KSTEP that can be used to "switch" the +5V signals like Start/Stop and For/Rev to GND. Try connecting the Spindle Signals to the Relay + and the Relay - to GND.
Toggling the Relay Driver IO (KFLOP IO 0 and 1) as described here under Relay Driver Outputs should activate the signals:
Using KStep

HTH
Regards