Export it and save it somewhere as PNG, open that file in an image editor that supports transparency and erase the body but leave the face textures alone, save as PNG and import back into sims 4 studio. When it opens up, you'll see that in the 'texture section' under 'diffuse' there will be a picture of a gray body texture.
It shows all the swatches separately so just select the ones you want to have a face-only overlay, hit next, and save your package to your game mods folder. Put the original overlay package in your sims 4 studio mods folder (documents\Sims 4 Studio\Mods) open Sims 4 Studio and under CAS select 'add cas part swatch.' Then, in the content filter at the top, select 'custom' and it should be right there. Yes, this is possible, and quite easy tbh.
