I don't think it is possible to add mustaches like I wanted... I wanted to add them just like people can add hair but it seems to be impossible to do that with the current code permissions (HeadDefs or something similar is needed, correct me if there's an alternative).
There are three options:
- Wait for the game to add the needed Defs;
- Add the mustaches as if they were hair (merging both textures), with the disadvantage of having no mustache visible when a hat is equipped;
- Replace the existing faces with ones with mustaches on. The disadvantage is that EVERY colonist with that face type (there are only a few...) will have a mustache...
There are three options:
- Wait for the game to add the needed Defs;
- Add the mustaches as if they were hair (merging both textures), with the disadvantage of having no mustache visible when a hat is equipped;
- Replace the existing faces with ones with mustaches on. The disadvantage is that EVERY colonist with that face type (there are only a few...) will have a mustache...

Thanks for that!