Combine Redux Reducers without add a new Level to Existing Reducers

I have a need to combine new reducer with the existing one, and the store hasn't configured with combineReducers yet, so this is the first it'll be configure.

Taking from the docs' example, I think this is how it'll be implemented:

import { combineReducers } from 'redux';
import existingReducers from './reducer';
import newReducer from './new-reducer';

const reducers = combineReducers({
  existing: existingReducers,
  newReducer
});

By that approach, now I need to add a new property in entire state which used the existing reducer. e.g

from state.user.email need to change into state.existing.user.email.

Is it possible to use combineReducers without adding a new level property?? I don't want to add a new reducer into the existing one anymore.

Existing State

{
  user: { email: 'abc', ... },
  foo: { ... },
  bar: { ... },
}

If using the above approach, I'll get this state

{
  existing: {
    user: { email: 'abc', ...},
    foo: { ... },
    bar: { ... }
  },
  newReducer: { ... }
}

Expected State when using combineReducers

{
  user: { email: 'abc', ... },
  foo: { ... },
  bar: { ... },
  newReducer: { ... }
}