how to add a passphrase to a keypair

[anonymous]

Jack via Gnupg-users <gnupg-users at gnupg.org> wrote:
> I may follow up on this later, but are you saying that if there is no
> password on the key, then gpg/gpg-agent/pinentry will not even prompt
> for it? So, if I did have a key without a password, then "gpg --passwd
> that-key" would not prompt for the original (blank) password, and only
> for the new password?

That is correct. It will only prompt to "enter new passphrase" if the key does not already have password protection.