How to get all objects from WINDOW (added from another classes)

I am working with specific program. It has "main window" (a.k.a. "form") where another extensions(protected dll, without access to their source code) can add some objects (texts, labels and etc..) in that window.

How can I access all the objects which is added on that "form" ?

here i example of typical extension:

....
namespace ProgramName.Extensions {

    public class MyExtensionName : ProgramName.ExtensionBase{

         //here goes typical codes to add things to "form" window,like:
         Draw.Arrow(MainFormObject, ArrowColor, ArrowWidth)

    }

}

what code I can use to access all "add"-ed elements , from other, anonymous/inaccessible classed? Is there any "reflection" or any solution?