Hello again !
I finally managed to connect my recalbox to the wifi, without a usb keyboard 
This is how I did it, in case anyone needs the instructions
- I put ubuntu on a usb stick, and boot it (I know, I know, it's overkill XD )
- I mounted the SD card containing recallbox os.
- From there, I could navigate inside recallbox filesystem, and open recalbox/share/system/recalbox.conf
- I edited my wifi key and SSID with the correct informations
- Plugged back the SD card in the Raspberry PI, and voila ! My recallbox is now visible on my local network
I don't know what happened with the ethernet cable though, it's probably a problem with my box 
Anyway, now I'm the happy owner of a fully featured retro gaming OS. It's amazing how much work the community has put into this thing !
Thank you !
PS: now I have to mess up with themes, roms, music, and... Arcade (seems complicated)
