I'd like to convert a skin overlay (cc) for personal use in my game, but I can't find any tutorials or tools anywhere for skin conversions, only clothes and hair.

Well, you can't convert skins other than extracting the textures from the TS4 skins and then copy/cut/pasting them over to the TS2 skins, and somehow make them look decent. The UVmapping and face structure is quite different between the two games, so the result might depend on that, plus whichever skills you have with the photo/painting program you use for the process.

It's essentially photoskinning.

Not sure if there are any 4t2 ones, but there are several for blending skins, which show some of the process.

(How to edit skins) https://www.youtube.com/watch?v=JgKd4A-NIAk
(Skin editing - but use TS4 skins for the details instead of celebrity pictures)
1- https://www.youtube.com/watch?v=Tg5Bt6KSrag
2- https://www.youtube.com/watch?v=1cufe1ftLc0

As for extracting them from TS4, the original ones can be found with the Sims4Studio (Create CAS standalone, part-type "head") although they do appear to have been dressed up in undies. Not sure if there's an undie-less version anywhere, or if they have to be extracted from the game files. If you want to extract CC skins, they can be opened with either S4Studiio or S4PE and extracted.

Aw, I was hoping there was an easier way other than just cutting and pasting lol. Thank you for your help tho!