You wouldn't transfer a passkey to your YubiKey. You would enroll it as another device that can be used to login. Like right now on my Google account I have an iCloud passkey and a Windows Hello passkey since iCloud can't sync a passkey to Windows (yet).
Hypothetical political journalist just lost both his Yubikeys when his office and home were both targeted. He still has an off-site backup of his passkey. How would you envision this working?
I'm unfamiliar with the particulars of passkeys, but with hardware tokens I've used before, you'd log in ASAP and unenroll the keys you've lost custody of.
