static class Hogeable {
　public interface IHogeable { Hogeable.HogeableObject HogeableObject { get; } }
　public class HogeableObject { public int Value { get; set; } }
　public static void Hoge(this IHogeable hoge) {
　　hoge.HogeableObject.Value += 1;
　　Console.WriteLine(hoge.HogeableObject.Value);
　}
}
class MyClass : MyBaseClass, Hogeable.IHogeable {
　private Hogeable.HogeableObject hogeableObject = new Hogeable.HogeableObject();
　Hogeable.HogeableObject Hogeable.IHogeable.HogeableObject { get { return hogeableObject; } }
}
//MyClass obj = new MyClass(); for (i=0;i<10;i++)obj.Hoge();
こうすればインスタンスフィールドも持てるんじゃね