The new MidiRig GM sound module is an excellent choice for an accordion or general purpose MIDI expander.
You can see more information here: http://midirig.com/products/sound-modules/gm-16-channel-sound-module/
It is super compact and easy to use with a wide variety of sounds.
Note that this expander needs to be set from a control panel on the accordion, from a PowerKey function or by a computer.
This enables setting and part changing.