To mod them to be available is quite simple.

Go to my computer > local disk > program files > activision > rome - total war > data > world > maps > campaign > imperial campaign > descr_strat

You'll see this:

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


Simply change this to:


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
romans_senate
slave
end
unlockable
end
nonplayable
end

Just don't click on the faction info (or was it senate) tab if you play as the Senate as the game will crash.