I could finally make my joystick and buttons work. It's not a very clean solution but it's working. In case people facing the same issue stroll around this post, here is what i did :
1) plug my usb encoder to my xubuntu (14.10) machine
2) install rejoystick
3) map the joystick axis and button to keyboard input
4) copy the the .rejoystickrc file stored in /home/myself, paste it and overwrite the one in /home/arcade on the target machine running GA (i couldnt use it in lxde on GA, no way to click on OK to save the mapping due to the low modeline)
5) edit the file .xinitrc with rejoystick -d
to start rejoystick daemon when booting
6) plug back the usb encoder
I believe that this is a dirty workaround to accomplish something that should work by editing ume.ini file, like it seems to for others...So, if someone could enlight me by sharing a better solution, i will be grateful.
Anyway, hope that helps !