What is the ideal way to use a foot controller (or other external source) to control the filter frequency (or other common params)? I do not see a native setting for pedal (I do see one for breath controller)?
Seems like I would need something like in the link below to send MIDI from my basic Moog expression pedal. But, even if I had one, is there a way to easily set up the external modulation in the Digitone?
You have five modulation sources per track/sound. Pitch bend, velocity, modulation wheel, breath controller and aftertouch. Pitch bend and velocity are pre-routed (to pitch and volume). However you could unlink them and use them as custom modulation sources. The other three modulation sources aren’t pre-routed to any parameter.
Per modulation source you can assaign four parameters, each with individual positive or negative depth. You can enter these settings with func+trig. - They get saved per sound patch.
Your mentioned “pedal to midi converter” to control would work perfectly.
You could even get the bigger version with four pedal inputs and control 4 of the 5 modulation sources 
One way to modulate internally is to P-lock FM parameters(TL-ML) and/or do a live record changing the TL and Multi very fast can do some sweet FM mod. I don’t own a digitone but this is how I modulate on my Sega Mega Drive MIdi.