I can do it when I'm at home (evening) and if someone is interested, but it is pretty straightforward. Just opened the Basesystems.xml, copied a line from the FB system and pasted changing the retroarch FB dll name for the Mame dll name.
Let me also mention that did another change in the Basesystems.xml. In the 3DS Citra emulator, I removed the parameter -f , otherwise the emulator won't open the game and will say that you are trying to open a rom with the wrong extension...
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