just to elaborate.....you want to dry hop after primary fermentation is complete, because as yeast produce c02 during active fermentation, the hop aroma that you intend to get from dry hopping will just blow out of the air lock. So adding more yeast: #1 probably wouldn't do anything since the sugar has already been consumed/converted, and ....#2 would be counterproductive to what you want to achieve from dry hopping.
