CtionScript 3

1.Make  a keyframe on the first frame of the main timeline of your movie and write:

var loader:Loader = new Loader();

loader.addEventListener(Event.COMPLETE, addThis);

function addThis(e:Event):void{

  addChild(e.currentTarget);

}

2.Make  a keyframe on the last frame of your MovieClip and write:

//replace external.swf with the actual name of the swf file you want to load

root.loader.load(new URLRequest("external.swf"));