@Stefan Welcome to the Rockstor community. In order to keep things as simple as possible Rockstor doesn’t support btrfs in partitions for the ‘data’ disks, only whole disk btrfs is catered for. The only exception to this is the root btrfs partition itself, and this has additional restrictions as a result. This really helps with simplifying things and gives us a nice clean ‘base line’ to analyse the disks from. There are no plans that I am aware of to change this as it would require a fairly major re-do of quite a bit of foundation code.
As for importing btrfs pools that is entirely possible as long as they are based on whole disk btrfs (as above) and have an existing label, that last one was a recently highlighted requirement reported by another forum member @AngeleToR and an issue has been opened to try and address this situation more elegantly.