I assume that this class is part of project you are working on.
There is one ethical problem here:
If in your company you see ownership of code as sacrum. Or worst you are disallowed to modify code that is not yours. Yes You may hurt feeling of person that maintain that class. Or angry him if his salary is related to his code.
so best is just to ask him for any help, and few suggestions eg. add getter function or change private to public - all based on your intuition of using that new code.
Hack that you have mention is really bed practice. If cant modify that code and maintainer dont wont to change anything. Consider using adapter design pattern.
