Hello,
Classical Capella viewpoints allows to create new elements and the generated code makes them already extensible. For example elements contributed by the Basic Viewpoints are extensible and can be extended in an other viewpoint.
About the Requirements Addon, this is not the same as it has been developed in a way that makes the Requirement elements not extending (subclassing) CapellaElement (and ExtensibleElement). The Viewpoint technology used in CapellaStudio is dependent and built on the use of ExtensibleElement instances.
So if you want to extend the classes from the Requirement Addon you cannot (at least directly) use the Capella Studio extension features. Maybe it can be done but it would require some investigations.
Regards,
Arnaud