You can use goat's milk in any recipe. I prefer to use it as half my water allowance. I use all types of milks (buttermilk, coconut, cream, half and half) I mix my lye in water and add the milk to my oils before the lye mixture or shortly after.
It just needs to be in the paste phase of making soap if you want to avoid using a preservative. I would not advise you use GM in the dilution phase without a preservative.