If you add more yeast, and there are more fermentables in there (there should be untill you hit 1.000 or below), fermentation will restart and you could carb naturally. Campden tablets dissipate after a day or two. But bear in mind that it will ferment completely dry if you do this. You'd have to wait for it to ferment completely dry, then add priming sugar and bottle. Or else you can watch the bottles carefully and pastuerize.
If you can force carb, you could just do that now.
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