I did some research, and here is what I came up with:
Apple juice contains different type of sugar, mainly: Fructose (~56% of the sugar), Glucose (~24% of the sugar) and Sucrose (~20% of the sugar).
Of course this may vary depending of which apples, but it gives us a good idea.
Fructose and sucrose are pretty much completely fermentable, and glucose (also known as dextrose) is 80% fermentable.
Knowing this, we can conclude that apple juice sugar is about 95% fermentable.
So in your example, if you have 50 grams of natural sugar in one litre of juice, about 47 grams are fermentable. Adding 53 grams of brewing sugar should bring your total to 100 grams of fermentable sugar.