In Rational Rose, in order to add an relationship between a future generated class and an existing class or between two future generated classes; shadow stereotyped classes which represent future generated classes needed to be created. The relationships could then be drawn between them.
In Rational Software Architect, when adding an relationship between a future generated class and a class in the model, you must add a relationship between the base classes and set theLeft_Class_Shadow_Type orRight_Class_Shadow_Type stereotype property depending on the direction of the relationship as per the Relationships General Tab.