This may work.
- Right-click on a package which includes Classes, and choose [Auto Create Class Diagram]
- Then a new class diagram will be created with all the models in the package
This menu is available per package. So if there are sub-packages in a package, it will be shown just as a package on the diagram. In this case, you have to auto-create diagram for every package and copy and paste Classes on the same diagram.
Do those classes have properties (such as attributes, operations and associations) set already? If not, there is another way that I can suggest.