Hi Mark,
You are correct a quadrature encoder is expected for Spindle Feedback. You can probably simulate an encoder with a User Program to monitor your single pulse and increment an Axis Position. See the SpindleOnePulse.c example.
The pulse will have to be long enough to be sampled reliably by the User Program. With 2 User Threads running that means it would need to be at least 270us long at the highest RPM. Alternatively you could use a RC (resistor+capacitor) circuit to delay the pulse by several microseconds so that the original pulse and the delayed pulse would create an A B quadrature type of signal that could be connected to a KFLOP Hardware encoder input to count pulses.
HTH
Regards