I'd pick up a gallon of <insert your favorite coolant brand here> and add it to the over flow tank until it is at the correct level.
Personally, I like Prestone. I always keep a gallon of 50/50 on hand and top off my / wife's car when I change oil.
Also, I bet in the owner's manual there is mention of adding coolant and the MFG requirements.
Unless you're tacking the car, I wouldn't worry about adding any additives to the coolant.