How to add new calendars?
You can add new calendars to your system and it will be recognized by NotePlan. You might need to restart NotePlan after adding a calendar.
If you don‘t see some calendars even after adding, please check the permissions. Sometimes you need both, read and write permissions and only read permissions won‘t be sufficient (in case of shared calendars via Google for example).
Add calendars on Mac
You can add new calendars on Mac using the system preference "Internet Accounts":
Add calendars on iOS
Open the Preferences app → Scroll down to and tap on "Calendar" → Accounts → Add Account: