Lee B wrote:

Is the variable speed a function of the fan itself or can that be
added at the switch?

It is a function of the motor, which will be a split-phase induction motor.
You either have a motor designed with multiple windings for multiple speeds,
or you don't. If you don't, the only practical way would be to replace the
motor with one designed with multiple windings, or get a new unit.

If you do have a motor designed with that feature, it will be a simple
manner of wiring up an appropriate switch to select between the speeds
available.

Jon