You can add only instances and pins to a cone. (Nets and ports, if required, are automatically added to the cone.)
To add objects to a cone:
Make sure the cone to which you want to add objects is the active cone.
In NetlistViewer or a Logical Cone view, select the instances and pins to add to the cone.
From the LogicalCone menu, choose Add To Active Cone> Add Selection.
All the objects appear in the active cone view. If the objects can be connected to other objects already present in the active cone, the command also connects those objects. If some of the selected objects are pins, the corresponding instances are added.