Creating Workspaces with Composite provider as the Central Provider

Hi Experts,

We are trying to create a workspace with a Composite provider in BW DSR which has joins. But when added to workspace we don't see options of any partitions under it to be marked as usable.

Is it limitation of Workspaces that a COmposite provider with joins cannot be added as a central provider in Workspace ??

Apprecite your advise on this topic.