You sure can as long as it doesnt have any sorbate or benzoate as a preservative and even then if its already fermenting it MIGHT not cause a problem. I hope this wasnt (isnt) a high end kit though as Im not sure that would be a great idea. I have no qualms with playing ari=ound with a cheaper 10 liter or 7.5 liter kiy though.
For flavor I would after you stabilize. Adding ti primary will just increase % w/ little flavor.
Like what Wade said tread carefully if a high end kit.
Better yet wait and just work with 1 gallon