How can I export html notes from Treepad?

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This and this page might be worth a look.

If you simply want to export your generated markdown to Joplin, you can use File -> Import -> Markdown (File).

If you want to preserve the hierarchy it gets difficult. I'm not an expert on the import feature, but I think it isn't possible OOTB. I guess there are two options:

  1. Split the HTML file in a folder/file hierarchy and convert to markdown. Then use File -> Import -> Markdown (Directory)
  2. Create the notebooks/notes by data API calls.