I created a brain wallet with a non standard phrase.
So I have that, the private and public keys.
Now this wallet app wants a 12 word recovery phrase to set up the wallet.
How can I convert what I have into a 12-word phrase that it can use?
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A short working example is worth a 1000 words.
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Use something like like ent to measure the entropy of you brainwallet text to ensure the text entropy is sufficient, and also apply some common sense to be certain search engines have not indexed your chosen brainwallet text seed. For example "Big Boys Race Our Young Girls But Violet Generally Wins" might satisfy a ent entropy test, but will fail a Google/Bing Test. Some other electrician or electrical engineer is likely to pick the same brainwallet seed phrase. Seed phrase uniqueness is an absolute must...
Firstly, it is highly recommended not to use a brainwallet, your coins will be stolen.
But in general, if you have a private key, there is no way to obtain the 12 word recovery phrase from it. That is because the 12 word phrase is used as a seed to generate a hierarchical deterministic master extended private key, as described in BIP 39 and BIP 32 and this process is not reversible. Also, your private key is not an extended private key which is needed for an HD wallet, so the best you can do is simply import that single address manually. Some wallets do not support importing of individual addresses for safety, because they wouldn't be covered by the 12 word mnemonic backup. So I would suggest just generating a new wallet and everything and forgetting about your brain wallet.
Echoing some of the previous comments, make absolutely certain that your brainwallet password/phrase is as strong as you can construct. It should be 30 characters or longer, with no, or very few, complete words; it should incorporate multiple sources in order to compound its strength; it should incorporate at least some personal information only you know.
All that being said, let's do this thing:
Create a hash of your password/passphrase, output in hexadecimal notation. You can do this using any number of tools, both online and offline.
Go to the BIP39 generator created by Ian Coleman. Select the radio button which reveals entropy details.
Copy/paste your hexadecimal hash output into the entropy box.
Select the dropdown menu item for Mnemonic length. 12 is the most common seed format.
The generator will now output a 12-word BIP39 mnemonic, which can be used to seed a wallet.
I strongly suggest that when you enter the mnemonic, you also select a password, which changes the seed master key without changing the mnemonic, allowing you to share the mnemonic without risking your funds. This password should also be insanely strong as well.
Once again, you had better be absolutely certain that your brainwallet password/passphrase is extremely strong, or it will get hijacked and you will lose your funds.
The Coinomi Wallet app allows you to "sweep" a paper wallet. This command appears on the context menu after the default wallet is created.
The sweep command allows the user to scan the private key to import the wallet.
The user could then transfer to an HD account if they wanted.
Coinomi is a safe app to use to claim your Bitcoin Cash or other Bitcoin forks after the fork. It may take a while for the currency to show up in the wallet after the actual fork. I am not connected with Coinomi.