Can I slow down extraction fan w/controller onto another controller?

I have a Vivosun extraction fan with a 3 speed controller already. Can I plug that into another fan controller to further slow down the speed? Or would the fan need to be controller-less?

There are different types of fan motors and different types of speed controllers. You will need to be more specific about the exact fan you have. If it's an AC motor with a 3 speed selector switch then you can very likely run that on a variac to further reduce speed. I have some wall mount oscillating fans (big ones) that have 3 speeds and they work on a variac allowing me to select the slow speed and then with the variac slow it down even more.

its a vivosun 440cfm with 3 speed selector. just H,M and L. I want to go lower than the preset low.