How to write extension method that work with base class

Idea is to have extension method that extend my functionality.

So instead of having something like this:

 return Add(Add(storage.GetFirst(), 3), 7);

I want have something like this:

return storage.GetFirst().Add(3).Add(7);

Problem with extension methods is that they have to be static in static class. This is simplify example of what I want to do.

 public class Storage
    {
        public int GetFirst()
        {
            return 100;
        }
        public int GetAll(int x, int y)
        {

            // ... 
            return x + y;
        }
    }
    public abstract class MyBase
    {
        protected Storage storage;
        protected MyBase()
        {
            storage = new Storage();
        }

        public int Add(int what, int howMuch)
        {
            return storage.GetAll(what, howMuch);
        }

    }

    public class MyClass : MyBase
    {
        public int method1()
        {
            return Add(Add(storage.GetFirst(), 3), 7);

            //I want have something like this:
            // return storage.GetFirst().Add(3).Add(7);
        }
    }

Off course classes Storage, MyBase and MyClass must not be static. Logic is simplify to have clean and simple example so relation between classes must stay same. What I want to do is to make Add method to be extension method, but leave everything else "more less same".

Is this possible to do and how?

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