adding definitions to namespace alias

Is it possible to add something to a namespace alias? Or how can I achieve the following:

using KdTree = ExternalLibrary::Special::KdTree;

namespace KdTree{
    class MySpezial {};
};

That does not work, but the following works:

using KdTree = ExternalLibrary::Special::KdTree;

namespace ExternalLibrary{ namespace Special { namespace KdTree{
    class MySpezial {};
}}};

Is the first method just prohibited by the standart?