NOTE:
I DO NOT recommend that you do any of this, it is purely for educational purposes
I am wondering if it's possible to create a mnemonic from an existing wallet.
For example, I got some existing paper wallets and want to create a mnemonic of that.
You would have to create an individual mnemonic for each wallet, you cannot create one mnemonic which will recover ALL of your paper wallets, which kind of defeats the purpose really... as you're still needing to keep a backup of some form for each wallet. The only advantage would be that it would be easier to backup/restore without transcription errors than a long bitcoin private key in WIF format.
Additionally, there is no "standard" way of converting a single private key to a mnemonic... you could use a "method" similar to BIP39, ie. using the 256bits from the private key as the "entropy", calculating the 8 bit checksum, and then break the 264 bits total into 11 bit chunks and then convert those chunks into 24 words from the BIP39 wordlist.
WARNING: if you put those 24 words into a BIP39 wallet, it won't generate your original private key/address... it'll generate something completely different!
As a worked example, we'll cheat and use
Ian Coleman's mnemonic code converter to do the "heavy lifting"...
Let's say that our Paper wallet was as follows:
(Compressed) Address: 19zAL47SZ1xBLiZmwiPt4fK9SSoxxCETfi
Private Key: L3Dt4emdX52UJ9R1gSghpR81sEVuXtv3r17iMRrCWQ1c1TLxCy3D
If we put that Private Key into
https://www.bitaddress.org/ we can see that the private key in hex is:
B31CAC51334D7E7E0197A4891B60D5F19481C9C58FE7FE7E3893E0544FD39AD9
Now using
IanColeman's Mnemonic Code Converter, we select the "Show Entropy Details" option:
We can then put in our 64 char hex private key in as 256bits of entropy:
And it automatically gives us a 24 word mnemonic:
reason tornado begin grief subject disease alien virus math swallow cube tobacco elite top bike woman wrist vault ceiling scheme eager truth hidden desert
However, as noted above... this mnemonic does NOT generate our private key/address:
Instead, to go from our mnemonic back to private key, you need to go in reverse using Ian Coleman's tool... paste the 24 words into the BIP39 mnemonic box, click the "show entropy details" box, then copy/paste the 64 char hex from the "entropy" box into the "Enter Private Key" box on the "wallet details" tab of bitaddress.org:
I DO NOT recommend that you do any of this, it is purely for educational purposes