If your accordion have 3 midi chanels (ch:1 right / ch:2 Chords / ch:3 Bass), you can easy use the SD4 for make a great sound module for accordion.
The Ketron SD4 have 3Modes to operate: Master Keyboard, Accordion, Organ, 2xMIDI.
In Accordion mode is ready to use! Maybe you have to make some sounds in other octave for de chords or bass, dependig the midi in accordion.
You have 64 presets sound, and more 64 user editable sounds (like Program with 3voices)