I made a test with Dropbox Paper.
So suppose I write documents, then export them in Markdown. (dropbox allow us to export them in that format)
Once it's done, I upload those markdown files to my (for example) https://www.dropbox.com/home/Applications/Joplin/letterbox folder) then I use jong_imp (a tool I use to import markdown files from dropbox to joplin) and that's it.
Jong_imp is just a script I configured in a crontab and the look for any file in https://www.dropbox.com/home/Applications/Joplin/letterbox folder and create joplin note.
Thus lovers of webapps can use Dropbox Paper and enjoy joplin 