Entirely plausible to add your own content. For a terrible guide:
Most of the parts are in .zip files, though it doesn't seem to matter if it's in a .zip or just a folder. Generally they have a battler, face, and sprite folder and a thumb.png. Thumb.png becomes the icon you click in the generator. Body also has a .json file. The only thing that really matters there is bodyIndex.
All other parts are named body#_[name of part]. The # indicates the bodyIndex it will show up in menus for. That does mean you have to make a unique zip for every body you want that part to fit on. They have to do it this way because of slight changes between the portrait (narrower chin, bigger forehead, different ear position, etc). The battler and sprite versions should be okay to remain the same across the board due to their size, but the portrait may require adjustment for each body type.
What about the parts themselves? Each part has unique requirements but the general rule is each .png is another layer, and the naming is strict for zOrder placement.