If the learn function on some (unfortunately not all) of Spark's controls isn't enough for you, you can always use a MIDI "sniffer" tool, to check what are the CC (or Note) MIDI sequences the controller is sending and then emulate them with your foot controller to achieve what you're after.
...although, I think you'll have an hard time convincing Spark to "listen" to your foot-controller on its native MIDI protocol mode instead of its other more generic mode that expects non-delta CC values only on those Learn-capable controls, but... maybe Arturia has more info about this than I do and that it can provide you.
You must also consider that this might be something Arturia isn't ready to support, something they are not willing to publish or make available to avoid unnecessary user queries, and support requests, etc...
