Adding Mutations to Vuex store as part of Vue Plugin

I'm creating a small Vue Plugin that allows a user to add a "page notification" from within any component. I've successfully implemented something like:

this.$notifications.add("a message");

And it works! But I've had to register the mutations and actions required for my plugin to work as part of the file that sets up the rest of the store for my app:

export default new Vuex.Store({...})

Is there a way to add actions and mutations to my store from within my plugin? It currently looks like this:

import vuex from './../store';

const MyPlugin = {

  install(Vue, options) {

    // 4. add an instance method
    Vue.prototype.$notifications =
    {
      notificationTypes: {
          0: "warning",
          1: "info"
      },
      add: function (options) {

        let id = "page-notification-" + (vuex.state.pageNotificationsCreated + 1);
        let message = options.message || options || "no message";
        let notificationType = this.notificationTypes[0];

        if(options.notificationType && Number.isInteger(options.notificationType)){
            // Map int to string type
            notificationType = this.notificationTypes[options.notificationType] || notificationType;
        }else{
            // Or use string we were provided ;)
            notificationType = options.notificationType;
        }

        let notification = { id: id, message: message, notificationType: notificationType };
        vuex.dispatch('addNotification', notification);
      }
    }

  }
};

export default MyPlugin;

Any and all help appreciated!