Well, as a minimum, one is going to have to add MIDI. Some people add MIDI just to the left hand so that they can add bass accompaniment but I am one of those that thinks that if you are going to add MIDI, add it to both side of the accordion. MIDI is the part that tells your expander what notes you are pressing so it can follow along with you.
Quite honestly, adding MIDI to a Verdi is likely a very expensive proposition. In some cases the MIDI parts and install could easily exceed the cost of the accordion itself.
In this case, you might be better off finding an accordion with MIDI installed or find a used V-Accordion which might cost more, but adds TONS more functionality. A V-accordion by itself might give you more than an acoustic accordion + expander i terms of sounds, but it has no rhythm section. It does have drum sounds though, that can be added to the bass side, so you make your own rhythm.