Note that it'll be a fullbody overlay, since it was a default skin.
The easiest way for me to make a new package for an overlay is to take an existing skin overlay package (of the same type, so one with a gray tones), drop it in your Sims 4 Studio mods folder (not the regular mods folder) and then open up S4S and select 'create cas standalone', select in the 'content' dropdown to custom and select the sample overlay you saved to the s4s mods folder, hit next, save, and import the texture overlay you want to make nondefault. Just open up the skin package and save the RLE textures you like.