You need to edit the respective game.xml to be found in the subfolder “games” of your TripleA map.
Best idea would be to study the xml of existing FFA-mods and compare what is different to “regular” game.xml.
Take as an example Napoleonic Empires 5 Player FFA:
here you see
<playerlist><player name="France" optional="false" canbedisabled="true"><player name="UnitedKingdom" optional="false" canbedisabled="true"><player name="AustrianEmpire" optional="false" canbedisabled="true"><player name="OttomanEmpire" optional="false" canbedisabled="true"><player name="Russia" optional="false" canbedisabled="true"><alliance player="UnitedKingdom" alliance="UnitedKingdom"><alliance player="Russia" alliance="Russia"><alliance player="AustrianEmpire" alliance="AustrianEmpire"><alliance player="France" alliance="France"><alliance player="OttomanEmpire" alliance="OttomanEmpire"></alliance></alliance></alliance></alliance></alliance></player></player></player></player></player></playerlist>There may be other parts of the xml affected.
HTH :-)