Although what your saying would work in theory, you will run into two problems. First the normal amount of tannin is very small depending on what your looking for, usually around 1/2 tbsp for the whole batch, this would leave you measuring very small tidious amounts for each bottle which you would have to incorporate by recorking and shaking your bottles thoroughly with new oxogine in the bottle although it wouldn't, ruin the wine, it's nit ideal for the longevity. Second is tannin should be put in during fermentation this aloes it to go through a few scientific processes that are beyond me, however the basics are it won't incorporate properly and you will have a tannic rich wine that needs a long aging period to soften. So officially there is no big problem from stoping you but my advice would be to hold off and enjoy the wine what what it is not all wines need tannic back bone to be great wine ! Cheers
