MonkeyC module nesting and extending Toybox

Is it possible to extend the standard Modules with additional functions and classes e.g. Toybox.System - possibly comparable to C# static extension methods?

In which way can I nest modules and what are the limitations? I can't find anything about module Nesting in MonkeyC

I had some trouble using this syntax in two different Source Code files with the same module names - that is why I'm asking the question:

module Generic {

  module Specific {

  }

}