Yes, you would add the Ceph presented S3 storage in under "Other". Once setup, you then set the tiering policy to snapshot only, and data that is only present in snapshots will be moved into it. Have a look at this page on our Documentation site for more information.
It is worth pointing out that this only moves the blocks which are used only in snapshots - it doesn't take each snapshot and send it to Ceph S3 as an object you can access independently.
StorageGRID is a NetApp product which provides S3 storage - when StorageGRID is used, there are not additional licensing costs as you have already paid for that tier to NetApp, but for other S3 platforms, there are.
So if I understand it correctly, simply said, there are active user data blocks, snapshot blocks and due to deduplication there are blocks used for both. And if I configure Fabric Pool with the "Snapshot only" policy, it will just move the snapshot blocks to S3 storage. Active user data blocks and deduplicated blocks used for snapshots and production data will still stay on the NetApp Storage (performance tier).