Organising model elements

There seem to be some restrictions as to what I can put in a tree underneath a class as opposed to a package. If I have a class, but also have an actor and system element of the same entity for different views, how can I organise them together in the model explorer? Is there a reason it isn’t letting me do this?

You may create a use case diagram as a subdiagram of the class, and then add the actor and system to the subdiagram.