You can import notes from other applications in Zettel Notes. To import notes, go to
Repositories > Long Press Repository > Import Notes.
Currently you can import notes only from StyleNote HTML file.
This application is not in active development (last update was in 2016). And users who used this application for many years are stuck with this application due to StyleNote being vendor specific.
This is another example of why one should think about future and should use plain text files for note taking. As plain text files will always be readable even if the note taking application is not updated anymore.