Import certificates into the System Keychain via the command line
Is it possible to copy a certificate to a machine running OS X Snow Leopard, add it to the System Keychain, then set it to "Always Trust via SSH?"
And if so... how?
To import a trusted certificate use the terminal command
sudo security add-trusted-cert -d -r trustRoot -k /Library/Keychains/System.keychain <certificate>
This will add a trusted certificate to the System.keychain. You should modify the options and paths to suit your situation. See the man page for security for more information.
↬ Rich Trouton, Adding new trusted root certificates to System.keychain