Normally this is not possible, a regular user cannot install anything in Recalbox.
hmm ok... too bad.
Anyway I tried Batocera, to see how this one handles the gamecube controller. Well, the controller works perfectly on Batocera. The C-stick is working. Unfortunately, I was unable to configure a specific config file for Gamecube as I did with Recalbox. I'm back on Recalbox now.
So, after all that, I forfeit. Too bad for that controller, I won't use it. I might look for an other one, better, more compatible. Maybe the one for the Nintendo Switch, it would be nice to use it Bluetooth.