Find passphrase using private key

It's actually a feature not to expose unencrypted keys. People would use it for simple backups without understanding the implications.

Just use ethkey generate random from your command line which is included with Parity:

secret:  42821fbb3b5110b0a5642313105dc9d7e521700ae26b702e10c248afa8486d0e
public:  cd455f9f4083b481f37fb96160505c77cb4f5cfe7759f29badf14489c35d44d62c2c9c6042aab23b2355fed36e632e652df8833ce5687a11b961af9495cb0355
address: d7ff0af0315c46e9ffb45513a5d69ab486754df7

And put it on ice. :)

I guess you can not generate seed phase from private key. But private key is everything for your account. So if had multiple accounts with some balance and you remember private keys for all addresses, then you are a lucky man.

If this is the case, you can simply create new account in metamask and get a new phrase. And then you can import your accounts to your metamask wallet using private Key.