Are there any aftermarket humidistats that can be added to Air Vent power fans that are currently only equipped with a thermostat?

It would be better to purchase a power fan equipped with a combination thermostat/humidistat than to add an aftermarket humidistat to a thermostat-only power fan.

If there are, they will not comply with the Underwriters Laboratory (UL) Listing carried by the Air Vent products. UL Listing is based on how the product is originally manufactured and made available to consumers. In order for the humidistat to work properly in conjunction with the thermostat, the line lead needs to be wired in parallel with the thermostat. Air Vent accounts for this during manufacturing of the product. In an aftermarket scenario, altering the wiring in the existing thermostat enclosure would be necessary, and that is not recommended. It would be better to purchase a power fan equipped with a combination thermostat/humidistat.