Is it possible to add your self-made parts like hats or masks to the creator yourself?

I would be interested in that Feature as well.

Currently that is not possible and it's not that easy to be honest. We work with different layers and an outfit alone is up to 5 layers. Then you would have to fit your items to every animation you use in every used direction and this for 7 frames each.

I may come up with a smart way to make the process easier in the future, but as of yet there's no ETA on it.

No problem, I can also add them to the spritesheets after exporting.