Just pick a fan type that you are not currently using in the applicable system. I think the auxiliary fan would be best - but you might need to test a few fan types (the ventilation fan MIGHT work).

Then, set that fan up (as you normally would - with the proper static and power - etc).

1. The trick here is to create a custom schedule - Create a schedule that is Off 24/7.
2. Then - set the reset and lockout so that the schedule is 100 percent, anytime the outdoor air enthalpy is less than 20 btu/lb (note that reset and lockout does not work for all fan types - thus, my earlier comment on needing to test it)
3. AND - add a rule so that the Room dry bulb temperature is set to ~1 degree less than your cooling setpoint (so that the economizer fan does not run in heating mode) I assumed the cooling setpoint was 75 here.
4. Don't enter anything for the offset

See the attached Image