I have used yogurt to help smooth out HP soap, even to smooth out CP soap that thickened too quickly because of an FO. It works better if your yogurt is not cold straight out of the refrigerator. Of course heating up the yogurt can make it separate, so it takes some planning to have slowly warmed up yogurt on hand.
But it's not only the yogurt that needs to be warm, everything that comes into contact with your soap batter (utensils, mold, FO, colorants) also need to be hot or as hot as is safe. Adding anything cold to a hot liquid will cool it down.
See this link
for additional tips to help with swirls in HP soap.