Extending a class that is a document class of a swf/fla

Hey
I have a question about extending or subclassing classes in as3 and flash.
I have a fla project that has its Document Class set to an as3 class called Portrait. B/c the portrait class is a document class of my fla project, it directly accesses all movieclip objects in the scene.

I want to create a new project that extends this project. By that, I mean I would like to extend the functionality by adding more movieclip objects to the scene and more script, but keep the original source code. Is this even possible?

I tried writting a new class called PortraitExt which extends the Portrait class. And I duplicated the original project, added some new objects, and set its document class to PortraitExt. However, when I tried compiling, it complained about the orignal Portrait class not being able to find the movieclip objects that are contained in the scene.

Thanks!