Mark,
The PLC supports some number of PWM (pulse width modulation) channels. You could use one of these channels as the source of pulses for your stepper driven fan. You can control the pulse (step rate) via the SetPWM statement in TBASIC.
SetPWM allows you to control the pulse rate and the duty cycle. For your application you need only 2 duty cycles, 50% and 0%. The 0% value stops the pulses.
I have used the PWM mechanism to control fan speed. My customer used a DC fan that accepted a PWM signal to control fan RPM.
Stepper motors are not, typically, used for fans for a lot of good reasons.
Fans used to cool CPU chips and mother boards are variable speed. The fans with 4 wires use a 0..5 VDC PWM signal to control fab speed. These fans are cheap and plentiful.
The PLC's PWM system will work with the 4 wire fans if you limit the signal to 0..5 V.
Best regards,
Gary D*ckinson