But i can't see the option to add a seed phrase to my private key.
Such option does not exist. The private key is derived from the
seed phrase not the other way around.
If i start a new wallet and use the seed phrase i have instead, can i make it bech32?
Although I don't recommend doing this but yes you can. You have to create a new wallet and before typing the seed words make sure to change the
seed type from
"Electrum" to
"bip39" then Electrum will ask you which address type you want to create, choose
"native segwit (p2wpkh)".