The libs that come with kicad are read only on purpose. An update of the software might wipe your changes. If you share your project that does use locally modified versions of the official lib, the person you share it with will not have the same libs. …
You can of course “install” your libs into your user folder. (Quite hard to achieve in kicad version 4.)
But do not add stuff to the official libs locally. Make personal libs instead.
If you discover a problem consider sharing your fix with others by making a pull request on the github repo responsible.