Open the descr_strat.txt, which lies insde your data/world/maps/campaign/imperial_campaign folder. The first lines should read:

Code:
campaign		imperial_campaign
playable
	romans_julii
	romans_brutii
	romans_scipii
end
unlockable
	egypt
	seleucid
	carthage
	parthia
	gauls
	germans
	britons
	greek_cities
end
nonplayable
	romans_senate
	macedon
	pontus
	armenia
	dacia
	numidia
	scythia
	spain
	thrace
	slave
end
Just cut + paste the factions you want from the nonplayble and unlockable sections into the playable section. To avoid some bugs, you should probably leave rebels and the senate as they are. So the final code would look like that:

Code:
campaign		imperial_campaign
playable
	romans_julii
	romans_brutii
	romans_scipii
	egypt
	seleucid
	carthage
	parthia
	gauls
	germans
	britons
	greek_cities
        macedon
	pontus
	armenia
	dacia
	numidia
	scythia
	spain
	thrace
end
unlockable
end
nonplayable
	romans_senate
	slave
end
The proceedure is the same in BI, but there you gotta be careful when it comes to emerging factions, as they require some more complex modding.