Since the Parts Tree and Connection Tree are loaded from an XML file, it was long anticipated that a feature would be added which allows users to replace the default tree. Now, that feature is available.
Right-click on the parts or connections tree allows replacement of the XML file for the tree. The new tree will not take effect until the next time TinkerCell starts.
The natural problems with this is that the heirarchy or parts depends on the tree structure, which can be problematic for some plugins that use the isA("...") to determine whether a part belongs to a particular family.