How to override base classes\structs such as int, string?

To reduce downvotes: (Skippable at first)

I am aware that this question sounds pointless and\or weird. I am creating JIT that takes C# code compiles it with csc.exe, extracts the IL and parallize it into CUDA, and I want to override some of the things in C#.

How override base things such as int\ string? I tried:

class String { /* In namespace System of course */
    BasicString wrap_to;
    public String( ) {
        wrap_to = new BasicString( 32 ); // capacity
    }
    public String( int Count ) {
        wrap_to = new BasicString( Count );
    }
    public char this[ int index ] {
        get { return wrap_to[ index ]; }
        set {
            if ( wrap_to.Handlers != 1 )
                wrap_to = new BasicString( wrap_to );
            wrap_to[ index ] = value;
        }
    }
    ...
}
... // not in namespace System now
class Test {
    public static void f(string s) { }
}

But when I tried:

Test.f( new string( ) );

It error'd Cannot implicitly convert type 'System.String' to 'string'. I tried moving my System.String to just string in the global scope and it error'd on the class itself. Any ideas how? I think somehow it could help if I can compile my .cs files without that mscorlib.dll, but I can't find a way to do it.

Even a way to reach to csc.exe source code may help. (This is critical.)