Extension method by type

Is there a way to create an extension method for a type? It seems I can only create them for instances.

public static class MyExtensions
{
    public static string Test(this string s)
    {
        return "test";
    }
}

public class Test
{
    static void TestIt()
    {
        string.Test();  // won't compile

        string s = null;
        s.Test();
    }
}