Slitherine

I want to enable Gallic warbands from the start, as having the regular Hellenic roster is a recipe for suicide against the elite Arverni warbands. What would I edit or add to allow Massalia to recruit a basic Gallic warband impact foot unit from the start?

You need to alter the 'Units' spreadsheet.

But the core problem is that warbands are not a Hellenic unit so if you add them so that Massalia can use them as a form of medium infantry then all the Hellenic powers will get them.

An alternative might be to add them as a provincial unit for where Massalia is located, but even so that forces you to create the province by some form of expansion

No, the simplest way is to add them as part of the nation roster of the faction, faction file.

I'm away a few days, so don't have access to the game files, but you should find for each faction 3 column, one are the units available at CL1, then another for CL2 and a third to CL3. Just add the units you want in the appropriate column. Anything is allowed, as long as you pick their reference from the units file.