You can add sounds using a sampler and midi sound module.
It is true that there is no DIRECT way to add your own sounds to the FR-18, but it can still be done.
If you are adept at such things, you can create your own samples of an internally mic'ed Cajun accordion, and then load them into a midi sound module. Connect the FR-18 to the sound module via a midi cable and voila!
Yes, it's probably a lot of work to do it well. And yes, the midi sound module is just one more piece of gear to haul around and set up. The point is, that it can in fact be done if you had enough determination to do it. I am not one of those people myself.
I don't know if Roland could be persuaded to take any action themselves. It is possible. I sent them a suggestion and I was pleased to see that they added that feature to the software update. It was a pretty obvious suggestion, and I don't know that they were influenced by what I said or not.
Re: Where can I purchase a 'Cajun Style Accordion' that can play in ALL keys?
Yes. I've been building one en cachette, that is actually has 11 buttons (so does not meet your specs), plays in all keys except Fb and E#, with bases to match. Treble has HiLo LoHi rows. Patent pending
Re: Where can I purchase a 'Cajun Style Accordion' that can play in ALL keys?
Be on the look out guys because I know Brandon Miller and I have been talking about this subject for a while. It is possible to have an accordion play in all keys