You found a bug in the code or need a new hook?
Simply create a new issue at https://github.com/pkp/pkp-lib/issues, desribe the change you need or want to make. You can also create a pull request via github to simplify the work for PKP. If you convinced them, they can simply approve the pull request and add the new feature to the master branch.
Here is an example for an issue with pull request: https://github.com/pkp/pkp-lib/issues/1500.