Command+click (on a Mac, I don't know what the equivalent modifier key is on a PC) on the 'Hold' buttons and a dialogue box will appear giving you the choice of MIDI controller or NRPN number to use as a controller, by either clicking on the 'Learn' button, or by clicking on the word 'none' (a list of MIDI control numbers appears) and choosing which controller you want to use.
While saying this, I've just tried it out on my copy of Jupiter-8V2 Standalone. The dialogue box appears but the controller does not actually work. Must be a bug.
I've just tried Jupiter-8V (version 1) and it works. You can assign a control numbers to the 'Hold' buttons. So it does look like version 2 contains a bug. Hopefully Arturia will fix this on their next update.