This is for generating brand new keys, not deriving a seed phrase for existing keys, correct?
Discussion
Correct. There's no NIP-06 compliant way to go in that direction. If you're going to get a bip39 mnemonic involved, you have to start with a new mnemonic and then derive a Nostr PK from it.
(you *can* stuff an existing Nostr PK into a mnemonic backup, but if you run it through NIP-06, you won't get the expected outcome)