Hi to all!
Well… I’m come back from my holiday in Ireland (two weeks in Ennis… WOW ) and I’ve find the accordion that I have buy on e-bay… an old Hohner Erika (not Erica).
Now I’ve a question: a my friend is an piano accordion maker (sometime box maker) and he can replace the reeds to convert to C#D tuning… but… the other 4 buttons?
Someone have an idea?
whatever you like! want a C or G or E on the push, D or F or A on the pull, etc…? i wonder if anyone can find out what Mairtin O’Connor has on his extra half-row.
Wow. The possibilities are endless, really. My inclination would be something like this:
Push E G B C
Pull D F A D
I.e. a D minor scale, set up opposite to how the scale ordinarily lays out on the two row. That way you have each note available in either direction; you can play full right-hand chords, etc. Or you could take a tip from certain 19-century concertina makers and use the extra buttons for sound effects. 
Since you’ve got absolute control over your keyboard layout, I would definitely recommend you get a low G in there somewhere. That way you get the full range of the fiddle. If it were me, I’d substitute the G for either the low F or F#.