Note: This procedure requires editing a game file; creating a backup copy of the file before proceeding is recommended. With a text editor, edit the "descr_strat" file in the "\activision\rome - total war\data\world\maps\campaign\imperial_campaign" folder. At the top of the page it will have something like this:

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
scythia
spain
thrace
numidia
slave
end

Change it to the following:

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