I think all the skintones arround are based on the originals from Maxis.
So you clone the original with bodyshop and use them as a base for your own.
Works the same as recoloring a outfit unless you want a skintone that has dxt3 formats wich are better.
In that case you need the knowledge to work with Simpe and import the textures manually.
Tutorial: Making Pixel-Perfect Skintones