Add an organ

What's different about the way you notate it?

What has it got to do with adding an organ??

Yiannis... You wrote:
...a way to add cajun accordion in the library of instruments so that i can notate a score using this instrument.

If you employ elements like staff, clef, notes in you score, then MuseScore can notate for cajun accordion; and, if preferred, any text can be edited to read 'Cajun Accordion' instead of 'Accordion'.

Because 'cajun accordion' does not appear in the library of instruments does'nt prevent notation. It only affects playback of the score.
To get a 'cajun accordion' sound, you would need a soundfont which includes that instrument.
See:
https://musescore.org/en/handbook/3/soundfonts-and-sfz-files

MuseScore's own GM (General MIDI) SoundFont contains these accordion sounds:
Accordion
Italian Accordion
Bandoneon (which may sound close to cajun)

So, you can notate a score and choose one of those accordions.

On the other hand...
If cajun accordion is notated with some kind of tablature or other symbolism (like 3 row accordion fingering) then please post an image.
In that case, a plugin may be possible. See:
https://musescore.org/en/handbook/3/plugins#what-are-they

Also, there are folks who simply use ABC text notation for cajun accordion - without playback though.

Regards, and welcome aboard.