Custom private key creation

You can set any password to your Steem account: https://steemit.com/security/@noisy/how-to-set-an-own-password-which-is-not-generated-by-steemit

you can have as many types of keys as you like, for example foo_bar key: https://steemit.com/security/@noisy/public-and-private-keys-how-to-generate-all-steem-user-s-keys-from-master-password-without-a-steemit-website-being-offline