As long as the content is legal, it is perfectly possible to create adult content with the app.
The premium stories are much more flexible and also allow pornographic texts as long as the context is there, i.e. try to let the overall story build up slowly so that the erotic components can gradually unfold and are in a meaningful context to the overall story. If it is not possible to continue a story because there is already too explicit content in the created chapter, simply try to continue the story one or two chapters earlier and tone it down a little.