Hi Urs,
As of now, there's no plugin implementing the WordPress integration; a one of the blogging integration we've got so far is thanks to ylc395 and his plugin based on GitHub Pages
In theory such published blogs could be imported to WordPress but I haven't tried it myself.
That said, there's no technical roadblock to develop such WordPress integration using Data API, so if you find someone to do it, or do it yourself, it should be relatively straightforward.
From the standpoint of extending Joplin export capability to other formats, it's not very likely to happen as it is usually a burden to support in the future.