Its in the dpack_0.pak file. You have to open it with a hex editor. Do a search for things like Carthage, playable, trebia, etc. You'll find a lot that don't apply to the demo (or do but not what you're looking for). The parts that you want to change are about 3/4 of the way down.

I just tried adding a unit of gauls to the trebia battle, and it didn't work. On the faction selection screen I saw that the unit was added but all I saw was a 'unit placeholder' outline, and when I tried to start the battle it took me back to the main menu. Not sure why, maybe because they're gauls, and the gauls aren't in the battle header, I dunno. But I added the unit info exactly as it appeared in the tutorial part, and even put in a location for them on the battlefield very near another carthaginian unit, exactly as it appeared in the file, just slightly moved over. No luck. I'll wait till others figure it out because I'm at my modding experience limit.