Sorry to hear about your perdicament, but its not so bad like you might think. A little inprovisment is all that is required. First off no more sugar is needed. The yeast that is in there will just have more to eat and cause more co2. Not a good idea in a bottle unless you want to make bottle bombs.
Next, I would hit it with a shot of c02, but I am thinking that you dont have a co2 tank, because you would have kegged it. If you dont have co2 available then put it in a warm place for the yeast can throw off a bit of co2 and protect the remaining beer from o2.
Next get a capper asap and finish bottling.
As a side thought if it happens again you could always saran wrap over the bottles and attach them with a rubberband or metal metal ties that come with garbage bags. Then all you do is poke a few holes in the top of the saranwrap for the co2 to excape. its also possible to use this method as a primary or a secondary, although I have never done a fermentation like this before, but I hear it does work.
hope this helps.
Beauty lies in the hands of the beer holder
Kegged: two heartednot, cascade, ale, pumpkin ale, rye ale.
Bottled: Mambo in your mouth,
Primary: imperal brown
Secondary: two hearednot, 2 fisted IPA, chimay white, breakfast stout