Just to be clear, are you asking about syncing passkeys across devices (ostensibly within the same ecosystem)? Or exporting passkeys between different ecosystems?

The typical use case for passkeys is that you can register multiple passkeys on an account, representing each authorized device. Each device can have its own distinct passkey provider. Services like 1Password allow you to sync passkeys between different devices, but those all stay within the 1Password ecosystem. 1Password also lets you export passwords and passkeys to different credential managers, but that's a one-off operation, presumably because you're leaving that ecosystem for another.