there is a patch to do it, or you could do it yourself in the descr_strat file (remember to copy the file first)

Code:
campaign		imperial_campaign
playable
	romans_julii
	romans_brutii
	romans_scipii
	macedon
end
unlockable
	egypt
	seleucid
	carthage
	parthia
	gauls
	germans
	britons
	greek_cities
end
nonplayable
	romans_senate
	pontus
	armenia
	dacia
	scythia
	spain
	thrace
	numidia
end