As already answered it is not possible to add the inheritance split in the web modeler. In fact there are several features that exist in the web modeler but not in the desktop modeler or vice versa. The WebModeler makes layouting a bit easier and also has a cleaner layout for the domain models and a basic theme editor. The desktop modeler comes with other functionalities and therefore more possibilities for development – it is my weapon of choice :)
Edit: when you are using the desktop modeler AND the web modeler, make sure to sync your project https://docs.mendix.com/web-modeler/general-sync-webmodeler-desktopmodeler
There is a lot that is not really available in webmodeler. I think the only workaround is to use the desktop modeler.
Unfortunately at the moment the web modeler is a bit lighter in features that the desktop one.
I believe that going forward more features will be added to it however in the short term ,if a feature is missing, all you can do is raise it on the idea forum and use the desktop modeler in the meantime.
Hope this helps