no expert - but I don't think this will be the end of the world. I had this problem with some wine i was amking, and i just used coll boiled water. Obviously waters it down a bit, but i guess it depends how much water you are adding as to how much it will be watered.
You should be ok topping it up with water but make sure you mix in the equivilant sugar content that would be in your apple juice before adding it to keep you on track for your target alcohol percentage :thumb: