Experience Builder: how to extend the root reducer?

[ToddJacobus]

I'm developing a custom widget for Experience Builder 1.6 and I would like to extend the root reducer like this:

const newRootReducer = combineReducers({
  existingSlice: existingSliceReducer,
  newSlice: newSliceReducer
})

store.replaceReducer(newRootReducer)

Where "existingSlice" is the existing, system reducers and "newSlice" is my own, custom reducer, similarly to how you would do this for code splitting.

I don't understand how to access "existingSlice", as in this example.  Thanks.