Ember combining route data

I have a situation where I have a route extending another route but will add more data to this new route. What made it tricky is we want to merge the new data with the existing base route.

I can do this synchronously in the model hook with

return Ember.merge(this._super(...arguments), {
  foo : 123
});

But how can I retrieve data asynchronously and still use this Ember.merge, ie something like

model(){
  return Ember.merge(this._super(...arguments), {
    foo : 123
    baa: store.get('smthElse')
  });
}

will give me back a model with baa as a promise object, not with its resolved value.

I tried returning the Ember.merge inside of the store.get('smthElse).then but the this._super(...arguments) threw undefined errors.