Hi there, you can import your existing pools without too much trouble, the process is very well outlined in the 6.12 release notes https://docs.unraid.net/unraid-os/release-notes/6.12.0/#zfs-pools; however, please be aware of some limitations:
Not all pool topologies are supported
Please be carefull about mountpoints, as those may change from whatever location you may have to /mnt/<pool_name>
Autotrim and compression can be configured for the entire pool
First level datasets will be imported as shares, you have to change some settings in the Shares section and select the most appropiate config for primary an secondary storage (See https://docs.unraid.net/unraid-os/release-notes/6.12.0/#share-storage-conceptual-change)
If you find any erros or have any issues please report.
Iker's post in Upgrading with a ZFS dataset in place was marked as the answer