How to export from Apple Notes?
NotePlan saves your notes as plain markdown files. So generally speaking, if you can export your notes as Markdown from another tool, you can import them into NotePlan.
Unfortunately, Apple Notes doesn't offer a convenient way to export your notes. However, you can use the external tool Exporter to get your notes exported as Markdown. And then copy the notes back into NotePlan. Read here how to find out how to import Markdown notes into NotePlan.
The caveat is that there might be problems with the images if the path is not as NotePlan expects it and completed tasks are not properly exported in the first place.
A workaround, which requires some more effort, is to manually copy & paste the notes from Apple Notes into NotePlan. Or at least the broken parts after using the Exporter app. If you select tasks in Apple Notes and copy & paste them directly to NotePlan, completed tasks will be properly copied as Markdown. Copying images works the same way, but you need to copy them individually into NotePlan.