If you have some DIY skills, this little wheel electronics from Doepfer might be your ticket:
I use it with a Yamaha FC-7. There's enough space to put it on the foot plate of the pedal. It has 4 inputs, so additional pedals or switches can be connected.
I have an 88es, and when I read your question I thought "just plug them in" as there's a volume pedal and sustain pedal jack on the 88es... But... Since it couldn't possibly be that easy I checked the m-audio site and sure enough, the 61es only has a connector for a sustain pedal, and not volume!!!
So, my advice, sell the 61es, buy an 88es, because any other solution will cost more than just buying an 88es...
Option #2, buy a Behringer FCB1010, go thru the hassle of programming it, and you get program recall via stomp box-like foot switches and two volume style pedals... These things are really pretty useful in a live rig...
Now... Let's say you score an 88es... Here's the gotcha... The volume pedal sends out midi controller #7, which is volume but doesn't work how you'd expect. What you want is midi controller #11, which is expression, usually it's used to control "volume" but it can be used for other things in a sound, not just volume... It's easy to change the midi controller number sent on the 88es, just have to do this every time you turn it on... I think it's advanced controls, tap f#2, a1 twice, and then c3... Might be an octave higher, I printed out the steps and labeled the 88es on the front panel, because it just necessary!
Yes, definitely absurd. Let's make a compact controller, but not give the kind of pedal options a synthesizer player would need to do their job. Some keyboard parts are impossible unless you're using a foot switch and expression pedal.
Option #1 makes sense ... I just don't need all the keys, though. If I did need something for piano music, then I would prefer something that felt more like a piano.
I'll check out option #2, though.
Yeah, I don't play piano parts on the 88es. Have and bring 88-key weighted hammer action for that. Turns out the extra keys on the 88es aren't just for notes that are heard. Key switching presets in EVB3, or switching from legato mode to poly... You can use those extra keys for all kinds of stuff, including patch up/down... That's the power of MainStage... If the song is really busy, there's more keys to map out different zones to...
I compensate for the lack of knobs and switches, and not having aftertouch either by having a novation remote sl61 mk2 in the live rig, no shortage of knobs, sliders after that!
So, my live rig consists of 3 controllers, plus the FCB...
The FCB1010 is cool, replaced three volume pedals and three sustains pedals with it. Replaced the standard ROM with the UNO chip, some gotchas in this as well, but so far with a little thinking and creative reassigning in MainStage and it all works. No USB MIDI on the thing, so I had to try several USB <> MIDI interfaces until I found one that worked that also supported external programming of the thing... But... I use it and it works, is heavy, stays put and I don't have to "chase" pedals moving around on the floor while I play!
Still amazed that m-audio thought there was any benefit to saving one input jack on that back panel, would have thought it was more economical to have the same assemble in both the 61 and the 88, with the only difference being number of keys...
Hope this all helps, ping if you have any questions!