Another idea would be to deliberately destroy the encrypted primary key material you upload to ProtonMail. I'd suggest setting the capabilities of the primary key to just Certify, not Sign. It could very well be that ProtonMail never tries to decrypt the encrypted primary private key then, because it is never asked to do a certification. And since you can only tell that the encrypted material has been destroyed once you actually try to decrypt it, it would never notice and chug on happily oblivious it has been lied to.
Oh, to answer the original question, you're looking for $ gpg --expert --full-gen-key and then option (13) Existing key. HTH, Peter. -- I use the GNU Privacy Guard (GnuPG) in combination with Enigmail. You can send me encrypted mail if you want some privacy. My key is available at <http://digitalbrains.com/2012/openpgp-key-peter>
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