You'll have to take it apart.  It has a regulator to set the voltage to the smoke unit.  A simple variable voltage module would replace the fixed voltage one in the car.

I went one step farther, I added an RF remote control to one of mine so I can turn it on and off without having to fumble with the switch or get it close to me on the layout.

GRJohn thanks for the suggestion which is beyond my technical abilities unfortunately.  What I was considering was a separate track with an on-off switch that would have the car vent for a while and then shut off but did not know if these cars in prototypical land vented only when being filled/emptied or when they were moving as part of a consist.  The youtube vids I have found all seem to be when the car is stationary

Well, I have the hard part done, I have the RF remote controls available.

The Universal Wireless Remote Control Topic

The key-fob remote on the left triggers the relay board that is installed in the tank car on the right.  This allows you to toggle the smoke on and off anytime the car is powered.

Here's mine in my tank car train.