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.
This article explains how to use this too to convert between the 2 formats: Can I convert the NGRAVE "Perfect Key" to 24 words?