Question about Ledger Nano X - Onyx black

I know that on hot wallets you can add chains manually, but I doubt that on a hard wallet this would be possible given the inherently high level of security.

Not sure if it exists...
It's also kind of pointless, "crypto USB sticks", called cold wallets or hardwallets, are more used for savings purposes rather than for temporary crypto storage.
To answer the question, Ledger is by far the best cold wallet on the market at the moment, but there is no guarantee that it works on all blockchains/currencies.

Indeed, what allows us to store a crypto "currency/currency" is the compatibility of the wallet with the chosen blockchain.