Hm, I have a PSD file I was using to edit some male clothing. It has the base skin in a base layer and the UV map on another layer above it as a guide. If you want I can send it to you
Just as a reminder, for all what I've read out there, there isn't a way to create a skin but you can make overlays placing the skin image over a skin complexion standalone mod that you can get on Sims 4 Studio or Color Magic.