Ember-simple-auth change cookie on the fly

I have an Ember application and I use ember-simple-auth for authentication. I want to change the cookie stored, that is related with the authentication, on the fly. The reason is a feature of impersonating another user. I have used

Ember.$.cookie

but it returns "undefined". How could I change the cookie, after I have signed in as user 'X', in order to impersonate user 'Y'?