I like the idea of topping up, I would surely do that. Just as a heads
up, grapes vary all over the place as to how much sugar and acid they
have in them, even from day to day and what side of a hill its on in
season. That white powdery stuff you see on grapes contains yeast, it
can go on it's own. When you drink wine very young, the yeast still
contributes to the flavor; I have no idea what to do there but i would
not use bread yeast.

If you want to drink it young the juice probably needs to be high in
sugar and low in acid; if it's the other way around, it might be like
drinking lemon juice. If it tastes really sweet, go for it; if it's
not so sweet and pretty tart; add sugar until it's noticably sweet.
(If you are not on that bike, get a hydrometer...))

Joe