How to extend an object of the anonymous class

I have class method:

public object MyMethod(object obj)
{
   // I want to add some new properties example "AddedProperty = true"
   // What must be here?
   // ...

   return extendedObject;
}

And:

var extendedObject = this.MyMethod( new {
   FirstProperty = "abcd",
   SecondProperty = 100 
});

Now the extendedObject has new properties. Help please.