no it is not possible to do such a thing. You can check out the Custom Elements example to see how we provide a new Palette to the modeler. In short, we replace it with our own through dependency injection, by defining it inside the custom elements module(folder) ´index.js´ file.
It is just a bit more cryptic due to dependency injection that needs the module { paletteProvider: ... } and the provider definition as a value [ 'value', customPaletteProvider ].