What is the best pattern for a package whose modifications are done in outside of it? -

I'm looking for a design pattern where it fits in these glasses:

1 - Say that you have an animal with a 'animal', 'cat' and 'dog' package animals.

2- You have to modify a method behavior in 'animal' (override in the 'cat' and 'dog' in the package) without making any modifications to animal source code.

I have found the Visitor Pattern for this purpose, but with that problem, you are not allowed to accept the Picasa source source code, including the 'Interacting with a Implementation' interface, though .

What do you suggest?

I'm not exactly sure about the imagination, but you use the decorator to class out the animal package Can create, which will use the structure to increase normal behavior.