When I use dry yeast, I always pitch it with the wort at the fermentation temperature. There is nothing wrong with topping of the fermenter before pitching the yeast. It is pretty much standard practice when doing partial boil brews, or if you need to add some water if you boiled off too much. If it does come down to adding more yeast, I would boil the water, cool to around 90F and then rehydrate the fresh yeast. As I said, it might be just sluggish to take off. wait a bit longer to see if it starts up. You might also try giving your fermenter a gentle swirl to get the yeast back in suspension if it has started to drop out, but don't shake it. Just a gentle swirl to get things mixed up again. I just did an IPA that started off with good fermentation, and then stalled out a little. A gentle swirl got the yeast back in suspension, and it dropped a few more gravity points to finish up.