Unit Testing class with cocos2d::UserDefault - How to read from custom XML file

I have a class that stores user preferences. When the class is created it retrieves data from the UserDefault file on iOS using the cocos2d::UserDefault functionality.

UserPref::ReadFromUserDefault() {
  _mx_username = cocos2d::UserDefault::getInstance()->getStringForKey("mx_username", "");
}

Is it possible to get cocos2d::UserDefault to point to a fake XML file of my own creation for the purposes of unit testing my class?

Thanks! GCB