One way would be to open the skin in the skin editor, activate the lines and create a screenshot. Paste this screenshot in your favorite paint program and create the new textures above the screenshot.

You should scale the image to get a width of 1024 pixel and try to keep the aspect ratio otherwise your new texture will be streched.

I guess this is what you wanted to do.
You wanted to export the UV Mapping instead of the texture, which seems not to be supported as direct menu option.