1. Use FireFox to initially access the site and to accept the untrusted certificate.
2. Use FireFox's certificate manager at ForeFox / Preferences... / Advanced / Encryption / View Certificates to export the certificate. Use the PEM encoding and make sure to name the file with a ".pem" extension.
3. Use Keychain Access to import the certificate
You should now be able to access the web site from within Safari.