Unlock System Keychain from Time Machine Backup

Somehow a block of WiFi passwords ("AirPort network password") disappeared from my System.keychain file.

I'd prefer to not replace the whole keychain with a backup, as the most recent backup is a week old and I'm not sure what might have changed since then. Instead I'd like to copy just the WiFi passwords from the backup to the default System.keychain.

I've restored a recent System.keychain to another folder, and I can add it to Keychain Access, and I can see all 90-odd WiFi passwords that I want. I just can't open any of them or drag them into the System.keychain. When I try, I'm prompted for the "keychain password" which is not my login password.

I think the password to use may be found in /var/db/SystemKey; however, I can't figure out how to use it. Thanks for insight!

I was searching for the same thing. I haven't figured out how to do this with Keychain Access or the systemkeychain cli (ie to make migrating logins from one keychain to another easy) but if you just need to recover passwords/keys and plain-text is okay for you, this will sort you out:

How to decrypt the system keychain from another mac at the command line