ActionScript 2.0 :: How To Implement Interface To Class Extending MovieClip

Is there any way of extending a class which is extending the movieClip class? Or generally a way to add extra methods to an allready existing class without having to add them in the actual class declaration. I was thinking of using Interfaces but how can I implement an Interface to a class which is extending a movieclip?

Code:
import interfaceName
class className implements interfaceName{
//stuff
}
But where do I put the code which extends the MovieClip?

Code:
class className extends MovieClip{
//
}