Your question isn't very clear.
If I understood you correctly, you can inherit your own class from the built in class and then use for example
new
keyword to override a method.
However, you cannot use this technique to replace a method with some other parameters that are different than original parameters, you can just 'replace' the original implementation. For more info see:
http://msdn.microsoft.com/en-us/library/51y09td4(v=vs.71).aspx#vclrfnew_newmodifier[
^].
You can add new methods for inherited classes that are not included in the original class and you can also use
extension methods[
^] to add functionality to a builtin class without having to derive a new class from it if that's what you're after.