Currently, popular TON wallet applications like
TONHUB do not support adding a passphrase to the mnemonic seed phrase when creating or restoring a wallet.
This means that if you develop a wallet application that allows users to generate a wallet using a mnemonic seed phrase and an additional passphrase, users may not be able to restore their wallets in TonKeeper or TONHUB directly.
However, you can still create your wallet application with the seed + passphrase feature and guide users to use your wallet app for restoring their wallets generated with the additional passphrase. It's important to inform users about this limitation and instruct them to use your application for wallet restoration, as using the seed phrase alone in other wallet applications will not recover their wallet correctly.
The TON ecosystem is still relatively new, and wallet applications are continuously improving and adding new features. It is possible that TonKeeper, TONHUB, or other TON wallet applications may add support for passphrases along with mnemonic seeds in the future.
Until then, users would need to use your wallet application for restoration purposes if they have created a wallet with a seed + passphrase in your app.