Move Secure Notes items from Keychain of one Mac to another?
Keychain Access.app > File > Add Keychain…



You got it half-way right.


Quit Keychain Access.app, just to be safe.
Locate your keychain file, named whatever.keychain-db as you mentioned.
Get a copy of that file onto your new Mac, perhaps via a thumb drive.
Put that file in your home folder, in Library folder, in Keychains folder.In other words: ~/Library/Keychains/your_keychain_name_goes_here.keychain-db
Open Keychain Access.app on that new Mac.
Choose File > Add Keychain….
In the file-picker dialog, choose the file you just placed in the Keychains folder.


We are not really importing or doing any processing here. We are merely informing the Keychain Access app about the existence of this additional keychain file.