index.js

import { createStore, combineReducers } from 'redux'

const createInjectableStore = (createStore) => {
  return (reducers) => {
    const store = createStore(combineReducers(reducers))
    const replace = () => {
      store.replaceReducer(combineReducers(reducers))
    }
    store.injectReducer = (key, reducer) => {
      reducers[key] = reducer
      replace()
    }
    store.removeReducer = (key) => {
      delete reducers[key]
      replace()
    }
    return store
  }
}

////////////////////////////////////////////////////////////
const reducer1 = () => 'one'
const reducer2 = () => 'two'

const store = createInjectableStore(createStore)({
  reducer1,
  reducer2
})

console.log(store.getState())

////////////////////////////////////////////////////////////
const reducer3 = () => 'injected!'
store.injectReducer('reducer3', reducer3)

console.log(store.getState())

////////////////////////////////////////////////////////////
const instanceId = 0

class CoolWidget extends React.Component {
  static contextTypes = {
    store: PropTypes.shape({
      injectReducer: PropTypes.func.isRequired,
      removeReducer: PropTypes.func.isRequired
    })
  }

  componentWillMount() {
    const { store } = this.context
    if (!store) {
      // make your own store, or just use component state
      // if the data doesn't need to be shared across
      // instances.
      //
      // The rest of the code in this fake component
      // assumes there is a store to use from context
    } else {
      this.reducerId = `widget:${++instanceId}`
      store.injectReducer(this.reducerId, widgetReducer)
      store.subscribe(() => this.setState({
        reducerState: store.getState()[this.reducerId]
      }))
    }
  }

  componentWillUnmount() {
    this.context.store.removeReducer(this.reducerId)
  }

  shouldComponentUpdate(nextProps, nextState) {
    return nextState.reducerState !== this.state.reducerState
  }

  handleClick() {
    // now you can dispatch, and it's plugged into the app
    this.context.store.dispatch(action)
  }
}