Would also like to see this feature. Or really any path to migrating in from more general notetaking apps (e.g. OneNote, Evernote, Google Docs, Joplin, Obsidian, Notion, Zoho, Bear). Obviously capturing all of them doesn't make sense but many of these already have some method to migrate between the others or export to HTML. There's been a large exodus from Evernote so there's lots of support for moving those .enex files around to other apps already.
As for getting into the walled garden of formats supported by journaling apps...
- Journey imports from Evernote
- Day One imports from JSON or CSV
- Diaro imports from Evernote, Dropbox Datastore (.csv)
- Diarly imports CSV, text files (.txt) or markdown files (.md)
I can't vouch for the quality of any of those imports, and obviously having to use another app as an intermediary isn't ideal.
More concerning to me is I don't see options to export Diarium into standard formats (new user so maybe I'm just overlooking that).