Suppose Bob receives a clear-signed message from Alice. Bob wants to add his own signature to this signed message, because he has verified the message contents and wants his friends (network of trust) know the message is authentic. But Bob also wants to leave Alice's signature so that the message keeps its authenticity for her network of trust.
gpg can sign a single message with multiple keys at once (
gpg --clear-sign -u key1 -u key2), but can you add a signature after signing and if so, is
gpg capable of doing that? If not is it because it's against the PGP specification and why?