If you just want to be able to give the control a tweak with your foot now and again, rather than needing to 'ride' it, the simplest solution is probably an option knob:
If you really want to be able to use an expression-pedal-like controller, you'll need to modify the pedal so that the part of the circuit that was routed through the potentiometer is routed out to a potentiometer providing a similar range of impedance in a (custom-built or modified) foot controller. It won't usually work with an off-the shelf expression pedal, and as Todd says in the comment, you won't be able to re-use the same expression pedal for different controls on different stompboxes.
I'm sure a mechanical solution would be possible too, or an electrical solution using a small motor (both of which could then be more generally usable with any knob on any pedal), but I'm not aware of anything available off-the-shelf and as a custom build I imagine they'd be harder to make than the custom expression pedal solution.
If you're really interested in getting a lot of realtime control under your feet, it's possible that you might want to look into using a configurable multi effect box, rather than traditional pedals.