I think in general, the markdown conversion and import is the easiest way to import notes, as @muzak said. But I doubt that it can be done in this case, since the note content is somewhere nested in the CSV/JSON file.
You will probably have to write your own importer. I couldn't find much information about the file formats. If you are aware of some documentation or can post the exported files here, maybe someone can help.
Related links:
- Ticktick CSV to ICS: Reddit - Dive into anything
- Other Joplin importers: Importing notes from other notebook applications