But i can't see the option to add a seed phrase to my private key.?
What do you mean by adding a seed phrase to your private key? Do you mean deriving the seed phrase from your private key? If so, that's not possible.
We generate a seed phrase and then we use that to genrate our private keys.
Op every Bitcoin address has it's own special seed phrase.
This is not true.
Each bitcoin address can be generated by a very big number of different seed phrases.