I have added powdered milk to M&P. First you have to dissolve the milk in a little hot water. In her book, SOAPMAKING THE NATURAL WAY, Rebecca Ittner, suggests the following. "Use 1-2 teaspoons per pound of soap. To help incorporate milk powder into melted soap, stir in a bit of hot water to the milk powder in a small bowl until you have a thick liquid. Strain the liquid to remove any lumps."