keytool is a useful utility for dealing with Java keystores, but it has a significant disadvantage: you can not export private keys with a certificate using keytool. Therefore, the only thing you can so is to add the certificate as a trustedCert into the keystore, but not as a keyEntry.
Obviously, this is easily possible through the programmatic interface, but that can be hasslesome at times. At http://couchpotato.net/pkeytool/ you can find a really nice little tool that allows you to extract the private key in a separate file, and then re-import the private key file and the cert into a new keystore.