How to enable a block cipher or hash algorithm for a keypair?

[Crest da Zoltral]

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Hash: SHA512

I searched any documentation i found on the net about how to edit keys,
but I didn't found a way to enable a different cipher or digest? With
`gpg --edit-key $key_id showpref` it's only possible to view the
preferences and `gpg --edit-key $key_id pref` seems only to print the
prefs in shorter harder to read form. So how can I enable Twofish and
SHA-512 (without overriding the preferences with --cipher-algo and
- --digest-algo)?
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