OOP / Extending String

How do I create a class that extends the String class?

Lets say I’ve created an ‘exString’ class that extends String.

In my project file I do the following

var test:exString = new exString();
test = "Hello World";
text.someExStringFunction();

I get a type mismatch in my “Hello World” assignment because Flash sees it as a String and not as an exString.