@snip3rm00n OK I did the trick by editing the Python generator file in the terminal

ssh root@192.168.0.xxx
mount -o remount, rw /
cd /usr/lib/python2.7/site-packages/configgen/generators/pisnes
vi pisnesGenerator.py

And re-mapping the keys

RECALBOX_TO_PBCS_BUTTON_MAPPING = \
    {
        "up": "UP",
        "down": "DOWN",
        "left": "LEFT",
        "right": "RIGHT",
        "a": "B",
        "b": "A",
...

But it got side effects to save/load states indeed
NB: expecting you have remap your emulationstation keys to a snes controller of course

Following this post
https://forum.recalbox.com/topic/19565/pisnes-fullscreen-since-last-update-6-1-1-dragonblaze

@snip3rm00n Yo i've also tried and also did not work here