You should not have to modify a core Nuxeo class. The good way of developing with Nuxeo si to contribute additional plugins that will get hooked to the core features. We try to make sure that everything that could require to be customised is configurable through an extension point, an adapter, a factory, etc.. Of course the product can always be improved and we might have missed a point, just mention it and we will see if we can make it more flexible. You can read this introduction for more information.