Import of foreign aggregates in cDot is not supported.
In 7-mode, the aggregate itself was self describing with respect to contents. It could be imported as any name, and the WAFL filesystem itself described volumes, LUNs, etc.
In cDot, the logical ownership and identity of objects is maintained in the cluster database. The aggregate does not contain the definition of the SVMs that would own the volumes, nor does it contain all the information about the volumes and other storage structures themselves as some of that is maintained in the cluster DB only. There is no supported functionality to merge that unknown state data with the existing cluster DB.
I've explored this topic myself with NetApp myself a lot - I have two separate clusters in one location which I really want to merge into a single cluster - a number of storage processes in my organization would work better if it were one cluster. Even with full access to the full cluster DB's of both clusters, NetApp does not have a process to manage this type of migration. I've been told that within NetApp they are actively working on an automated process for this type of action as it is one of the last useful and functional things they can't do compared to 7-mode. The issues come into play making it bullet proof and automatic for end users.