You are not able to add a virtual attribute for the Status Report object (for example, a custom virtual attribute "My Virtual Custom String" referencing a custom String attribute in the Project object "My Custom String").
All supported Clarity releases.
Status Reports Workspace does not support Virtual attributes.
This is working as per the intended design.
Workaround:
As this is a cross-object workspace, add the non-virtual attribute (Project's "My Custom String") instead of Status Reports' "My Virtual Custom String".