If you want to modify the algorithm them why stop at such a simple change and not make a better change that can actually give you a proper security?
You have a mnemonic that is n-bits (eg. 128-bits for 12 words) take those bits and encrypt them with AES256 with a key derived from a strong passphrase and a salt derived from the address (like BIP38).
Now you get 128-bits of encrypted data (encryption without IV) which you can encode to 12 words using the same BIP39 scheme. All you have to do is write down these words as if they were your mnemonic.
To import to a wallet you just decrypt these words and convert to an un-encrypted BIP39. That is decode 12 words to get the byte[], derive the AES key from the passphrase and address like above, decrypt using AES256. Now you have un-encrypted 128-bit entropy. Encode this using BIP39 scheme and you have the original words.
What if a head injury or senility prevents you from remembering what you did somewhere down the line? You will have to explain that system to your descendants or write down exactly what you did. If someone discovers all that and figures out your system, your security isn't any better than what it would have been if you used a seed extension (passphrase) in the eyes of the thief. Assuming he knows what he found.