This is the brioche bun recipe I frequnetly use:  250 gr potato puree  3.5 cups flour  1 tsp sugar  0.5 tsp salt  2 tsp instant yeast  1 tbsp olive oil  50 gr butter  1/3 cup milk  1 egg  1 tsp bread improver  First mix the dry ingredients, then add melted butter, egg and potato puree. Then add olive oil and milk.   I followed the instructions but forgot to add instant yeast. And with the mistake remaining unidentified I waited for the dough to rise. But after two or so hours, all of a sudden I figured I didn't add yeat to the dough.   Is it possible to corporate it into the dough now? What can be done to save the dough? 
I will sometimes add yeast late, for instance when I'm not finding a sourdough culture to be adequate for a decent rise, or on the rare occasion when I'm getting poor performance from a particular batch of yeast.   I typically dissolve it in a small amount of water (or milk for this recipe), work the mixture into the dough, and add a bit of flour if needed to compensate for the added liquid.   It works. 