How to import Notes.app notes into Simplenotes.app?
Answer from SimpleNote Agent

Spoiler alert: Not unless you want to copy/paste. If you're a premium user of SimpleNote, you can email all of your Notes.app notes and import them that way.

The only other solution is if you can find a way to import a SQLite database into SimpleNote, in which case you would use the files located at:

~/Library/Containers/com.apple.Notes/Data/Library/Notes/