Actionscript 3 :: Overriding Public Method In Dynamically Loaded Class And GetDefinitionByName()

I have two SWFs: main.swf and external.swf. main.swf needs to access some methods in external.swf, so it loads external.swf into itself and uses getDefinitionByName("package.Class") to access the class and one of its methods:

var ExternalClass = getDefinitionByName("package.Class") as Class;
var ClassInstance = new ExternalClass();
var NeededFunction:Function = ClassInstance["NeededFunction"] as Function;
var response:String = NeededFunction(param);

Now, I need to extend the functionality of NeededFunction (which is a public method)... I know it's possible to override public methods, but how would I go about this with a dynamically loaded class?

I was thinking I could do something like this, but it doesn't work:

var ClassInstance["NeededFunction"] = function(param1:uint):String {
var newString = "Your number is: "+param1.toString(); //New functionality
return newString;
}