You can take the notes that you’ve created in other apps, and import them into the Notes app in macOS High Sierra.
When you import notes, you can add them to your iCloud notes account or your On My Mac account. If you store them in iCloud, you can automatically see all of your notes on any device where you’re signed in with your Apple ID.
- Open your Notes app.
- Choose to store your notes in iCloud or On My Mac. Click a folder in the account that you want to use.
- In the menu bar, click File > Import.
- Select the file or folder that you want to import. If the notes that you're importing are organized in folders, click Options in the lower-left corner to keep them organized.
- Click Import. When you see a confirmation message, click Import again.
- After your notes import, you'll see a new folder in the Notes app called Imported Notes. Then you can organize them into any Notes folder that you want.
(This how-to is based on my experiences and info on Apple's support pages — where the images sometimes come from.)