[7thsky™]

Старым проверенным способом

var t:Object = MovieClip.prototype;
t.applyClass = function (ClassConstructor:Function):Boolean {
	//check
	if (typeof ClassConstructor != 'function') {
		return false;
	} // end if

	this.__proto__ = ClassConstructor.prototype;
	this.constructor = ClassConstructor;
	this.constructor.apply (this);
	return Boolean (ClassConstructor (this));
}

// использование
some_mc.applyClass (Your_Super_Class_extends_MovieClip);