You do not have to encrypt the seed phrase because you can simply create a BIP39 passphrase. In this case, the seed phrase itself is useless without the additional passphrase, which you can write down and store in a safe location separately from the seed phrase.
How do i create a passphrase for my wallet which i have already created?
You cannot; you will have to create a new wallet.
Basically, if you add a passphrase to your current seed phrase, it will generate a new wallet with completely different addresses. This is another advantage of a BIP39 passphrase: you can have two (or more) wallets with the same seed phrase. One can serve as a decoy (with a small amount of coins), and the other, kept secret, can be your main hodl wallet. So, if someone gets their hands on your seed phrase, all they will be able to access is your decoy wallet with a small amount of coins. Your main stash in the secret wallet remains safe.
What is a BIP39 Passphrase?
https://www.blockplate.com/blogs/blockplate/what-is-a-bip39-passphrase