I found this for using a standard soap into a neem soap recipe. I think they call it melt and mix. And neem oil is sold on the same site as well. It looks like there is neem powder as well, I'm not sure if there is a benefit to the oil over the powder. Plus you could add a lovely scent!
Cut off whatever amount of soap you want to use.
Weigh it, and then weigh out 5 percent neem oil. If you have a 500 gram chunk of soap you add 25 grams of neem oil.
Melt the soap in a bowl in the microwave or a double boiler. If using microwave stir frequently and make sure it doesn't overheat or boil.
Stir in neem oil.
You can also stir in colors, fragrances or essential oils.
Pour into molds.