Perhaps the game will allow scripting, in which case you could force them to attack certain places.
Or like in MTW there will be a faction mentality setting which could be set to BARBARIAN_RAIDER or some kind of religious fanatic setting to make them hate the Greeks.
If the game will allow it (I'm sure it will) I'd move the campaign map Eastward anway, since provinces west of Italy aren't really of concern and perhaps south too, since the far north isn't really relevant for this period either. This way more of the Persian empire and Mesopatamia could be covered. More lands for Alexander to eventually conquer.