There's an easy way to do it with Bodyshop, you just export my skintone that has the size you want, and replace the textures with the ones you want to use. There's a tutorial here:
http://warlokkstower.googlepages.com/bodyshapeguideIncidentally, skintones reference the nude Top and Bottom Separate meshes, so you have to have those installed, of course... the full-body nudes are only used for outfits like swimsuits or undies. If you use that tutorial though you don't have to worry about it, it will do it automatically based on the size you start with.
