In object-oriented programming there are two approaches to accomplish what you are trying to do.  In approach 1, you take the base class and tweak one of it's methods through overloading.  In approach 2 you inherit from a base class and extend the functionality of the base class with whatever behavior you want.

For core objects, Matlab and Octave are slightly different.  Apparently Matlab treats its core objects as just another class and allows overloading.  The same strategy does not work for Octave.  I've changed the bug report Item Group to Matlab Compatibility to reflect that.  But this is so deep in the internals of Octave that I don't see it being changed for years, if ever.

On the other hand, you can get the same behavior (a new definition for '+' which handles cell arrays) through approach 2.

In order to do that you just need a new class with a basic constructor that wraps a cell array and a new function 'plus' that implements your extended behavior.

I've attached an example class called @myclass.  If you untar the directory somewhere and then start Octave in the directory above @myclass you should be able to see how it works.

Here is a sample running the example you posted.

octave:4> A = mycell ([1 2 3], [4 5], 6);
octave:5> B = mycell (5, [4 5], 2);
octave:6> A + B

{
  [1,1] =

     6   7   8

  [1,2] =

      8   10

  [1,3] =  8
}

Note that I did this quickly as a proof of concept.  If you really go this direction then you should use a better class name and maybe put in some checks on what the inputs are, etc.