How do I convert or load a UIImage into a PKDrawing?

I have the same question- I would love to adopt PencilKit, frankly because my drawing algorithm *****. That said, i'd need to load my current markup image into the PKDrawing in order to convert from one method to the other without losing all the user's work. I don't want to just place it behind the drawing as a layer, as they wouldn't be able to erase things.