Until we come up with an efficient way to handle the extensive hierarchies that are often included in CAD geometry files, the current workaround is to save the subassemblies you wish to interact with separately from the rest of the model as individual OBJ files. Once this is done, all associated OBJ files can be imported at once by selecting multiple files from the import browser. Once imported, interactive hierarchies can be created using the PICK PARENT command from the pop-up menu that appears when right-clicking on an object or the OBJECT -> PICK PARENT command from the main menu.
Note that there are two methods of picking available from the TOOLBOX. The default option with the single arrow always selects the top node of any given hierarchy regardless of what is selected when geometry is picked from within the 3D View window. The other picking option, the one with 2 arrows (one smaller than the other), allows subassemblies to be selected within the 3D View window.
Also, regardless of which 3D view picking option is selected, subassemblies can always be selected from within a hierarchy from the EXPERT PANE.