Can they add additional charges?
I would think things are rather fluid right now. I don’t see any reason why they couldn’t.
Once you file charges, you aren’t limited to them by the law.
If you can come up with other charges that apply, you could certainly file more charges this early on.
If it were a week before hearings started, the judge would probably look negatively upon the tactic.
I’m just expressing my observations and logic trail here though. I am not an attorney or involved in a position where I would know the law well.