[eridius]

The general answer to "why can't I modify my own OS installation?" is "if you could, so could malware authors".

[kuschku]

No, it is not.

That's why I asked if there was a way to add your own key to the keyring.

On Android, I can add my own keys to the bootloaders allowed keys, and lock the bootloader again. As is the recommended installation type for copperhead OS.