Here is a proposal



MooseTask
        MooseAbstractImporter (which right now is not used and only contain error handling and importingContext)
                           this class could hold the plugin mechanism that would be run as a tearDown from now.

              
                  MooseImportingTask - Renamed SmalltalkImportingTask (add importerClass logic)
                             MooseCompositeImporterTask 
                                  MooseSqueakClassPackageImporterTask -> MoosePharoClassPackageExtractTask
                                  MooseSqueakClasssCategoriesImporterTask - >MoosePharoClassCategoriesExtractTask
                             MooseImportClassesTask -> MooseExtractClassesTask

                  FileStructureImporter(/Extractor)


         MooseOperator


Since we will not be able to inherit from the same superclass MSEReader and MooseAbstractImporter.
We could define one trait to represent the plugin management and apply to the MSEReader and MooseAbstractImporter.

So let us know what you think.

Stef and Usman