I always add yeast nutrient to my starters and boils and I think the amounts typically used is not enough for higher gravity beers, but adding more nutrient to the boil isn't necessarily beneficial to the yeast. Therefore, you add the Fermaid K to the fermentor once the yeast is working. I suppose you could just add more of whatever yeast nutrient you're using.
Here's what the manufacturer sez: Fermaid K Servomyces