Tannin is usually added to the must before fermentation begins, but this is not absolutely necessary and, in your case, not even desired. We add it to most non-grape wines before fermenting, but often adjust it upward by taste, just before bottling. This can be a delicate process.
When the wine has cleared and is no longer forming sediments, sample it. The tannic bite is on the tip of the tongue and easily identified. If not adequate to your taste, add just a bit (1/16 of a tsp per gal) and gently stir with a glass rod or wooden dowel. Refit the airlock and let set about an hour and taste again. If still not adequate, add another 1/16 tsp per gal, stir, and let set another hour before tasting. If you think you’re almost but not quite there, add even less next time. By adding just a bit at a time, you’ll soon be able to taste the threshold you seek without a high risk of overdoing it. With most grape wines, it shouldn’t take too much to boost the tannin to your taste.
Another technique we’ve heard of to boost tannin and aid a sluggish finishing fermentation at the same time is to add a few (10-20) young, green oak leaves to the wine as its specific gravity drops below 1.000. Crush the leaves into a ball, put them in a sterilized nylon or muslin bag with 2-3 marbles (NOT steelies), tie the bag closed, and drop the bag in the carboy. Pin the tie-string between the carboy and bung for easy retrieval later. Leave this in the wine for about 10-14 days, removing when the taste is right. The leaves seem to reactivate the yeast and the wine usually ferments to dryness during that time.