>Do you consider that is breaking the principal of OOP ?

Yes, absolutely! The two fundamental benefits of OOP over procedural programming are Re-Usability and Maintainability.

If you add methods to objects (as opposed to classes) you cannot re-use the methods, or the code. In your example you cannot re-use your "added_method" anywhere else. So if the next form you create also needs it, you have to duplicate the code. Now, if you happen to want to change the code, you have TWO places that have the same code in them.

You also break the maintainability because you can no longer replace the object with another without breaking your code unless you first adding all the 'extra' methods back. Suppose you have a button on a form, to which you have added an extra method and then you need to replace the button with one based on a different sub-class. YOu now have to re-create the custom method.

What you are doing is NOT OOP it is simply procedural programming using VFP Base Classes directly. Instead of simply asking for changes in the tool, maybe you should consider whether the way in which you are trying to use it is actually correct?