security add-trusted-cert

security add-trusted-cert -d -r trustRoot -k /Library/Keychains/System.keychain YOUR_Certificate
(no output)

security add-trusted-cert -d -r trustRoot -k $HOME/Library/Keychains/login.keychain YOUR_Certificate
(no output)

https://apple.stackexchange.com/questions/80623/import-certificates-into-the-system-keychain-via-the-command-line

See also

• security (command), security list-keychains, security unlock-keychain, security find-identity, security add-trusted-cert, /Library/Keychains/, security set-keychain-password