for the Nomads GotM, you should open your alpha.txt file in the main game directory, search for the #FACTIONS section which should look like this:
#FACTIONS
GAIANS, GAIANS
HIVE, HIVE
UNIV, UNIV
MORGAN, MORGAN
SPARTANS, SPARTANS
BELIEVE, BELIEVE
PEACE, PEACE
and replace it with:
#FACTIONS
GAIANS, GAIANS
NOMADS, NOMADS
UNIV, UNIV
MORGAN, MORGAN
SPARTANS, SPARTANS
BELIEVE, BELIEVE
PEACE, PEACE
once you have finished playing the scenario, you just have to revert to the original.