You can only store a private key in hexadecimal format on the GRAPHENE. It is not possible to store a 12- or 24-word mnemonic seed on it, as the top plate contains scrambled hexadecimal characters.
But you can convert a mnemonic seed to hexadecimal format using the built-in tool in the ZERO or a tool such as https://iancoleman.online/. To ensure your keys don't get compromised, we recommend downloading this tool to your computer and only use it offline to convert your private key.