For use with an existing account, all you need to do is grab the public key generated by FairyWallet. There is no need to continue to the “create new account” step within the wallet.
You can change an accounts permissions with the EOS-Voter Wallet by Greymass. Log in to the EOS-Voter wallet with the Active key for the account you wish to change the permission on. Visit the “tools” tab at the top of the wallet.
Copy the public key you saved to a note in step three and paste it into the section for modifying the Active Key on the account. This transaction should submit very quickly. To confirm the change has been made properly visit Bloks.io block explorer, search your account name, and view the “permissions” drop-down. If you see the new public key assigned to the Active Permission then it has been changed successfully.
NOTE: It is not recommended to change your accounts Owner permission with this method yet as you will not have access to the unencrypted private key string. The FairyWallet doesn’t currently support changing permissions. The wallet may be implementing this feature in the future though at which point it would be safe to change your Owner permission to link with the Ledger/FairyWallet option using the same public key.
Never discard your genesis private key even if you change the permissions to new keys. You may need it for future forks that use the genesis snapshot accounts and the public keys that were associated with them.