NSQL should be able to read data from the DWH schema - you would just need to make sure (at database level) that the "Clarity database" can see the "DWH database" - if your are on-premise then this is something your local DBA can sort out (via DB-links for example, but not the only way), if you are on-demand I don't know whether that is possible (perhaps you define an additional database-source in the CSA and can do it that way too).