Siggi on discord helped,thanks.
Just post here for anyone with same confusion.
You cannot convert a private key to a mnemonic phrase. A private key is derived from the mnemonic phrase using a one way hashing algorithm. You have to set up a wallet using a randomly generated mnemonic phrase first, and then you can import your private key from Mist.
Keep in mind that your imported key isn't protected by the mnemonic phrase, if you lose your computer, HDD crashes, or anything like that, you need to have the Mist private key saved in addition to the mnemonic generated by metamask.
