Can I add a passphrase to the NGRAVE "Perfect Key"?

No, but we will add this in the near future.

A passphrase/password typically adds a second layer of security to the mnemonic phrase as an extra security measure in case your backup gets exposed. It is often called the "25th word" of a mnemonic phrase, but it can be added to the hexadecimal "Perfect Key" just as well. Since every password generates a valid seed and will unlock a different account, there is no "wrong" password. 

You can add as many passwords as you want to create different wallets. You could use this to create separate accounts for your family members, for instance. Or a business and personal account.

Adding a password can also be used as a way to protect you from the so-called "$5 wrench attack" ( and enables you to create a decoy wallet which has the same key as your backup but uses a different password. In case someone forces you to give them access to your wallet, you could provide the decoy password, and keep the real password (with all your funds) a secret.