Custom class extending MovieClip - constructor/assignment problem

Hello,

I've got a simple class:

package somepackage

{

import flash.display.MovieClip;

public class NewMc extends MovieClip
{
  public var myVar:String = "something";
}
}

And there is a question now: how can I assign a "real" MovieClip object to my created-class object?

I mean:

var realMc:MovieClip = new MovieClip();

var myMc:NewMc = realMc;

I think it should be possible, as the custom class extends the original class.

I tried myMc = realMc as MovieClip, but such conversion isn't allowed.

And one thing! I don't want to add the realMc object as the myMc child! This solution is obvious, but It's not what I want. My aim is to have a "first-level" MovieClip object just having additional parameters.

Any ideas will be appreciated