Some virtual instrument and effects parameters just cry out for footpedal control—too bad you don’t have a pedal that outputs MIDI data…or do you?
If you have a keyboard synthesizer or controller, it will probably have an expression pedal jack. The standard MIDI controller for expression is controller #11, and unless your keyboard or controller is really old, the odds are good that plugging an expression pedal into the pedal jack, then moving the pedal, will send controller #11 messages out the keyboard or controller’s MIDI out. A floor multieffects for guitar that has a pedal may also transmit controller messages.
The Yamaha FC-7 Expression Pedal can control more than just parameters inside a hardware synthesizer.
Assuming the target parameter you want to control has MIDI Learn, enable it (often done by right-clicking or shift-clicking on a control and choosing MIDI Learn), wiggle the footpedal, and now the parameter has “learned” to respond to your footpedal motion. Note that if another parameter is already controlled by controller #11, you’ll probably want to click on it and call up “MIDI Forget.”