//чужой бинарник
    public class BaseBinary
    {
        public void OldMethod() { }
        public void NewMethod() { } //2012
    }

//твой код и бинарник
    public class YouBinary : BaseBinary 
    {
        public void NewMethod() { } //2011
        public void YourMethod() { }
    }

//чужой код
    public class FinalBinary : YouBinary
    {
        public void DoSomething(){ }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var fb = new FinalBinary();
            fb.OldMethod();
            fb.YourMethod();
            fb.NewMethod();
            fb.DoSomething();
        }
    }