There is nothing wrong with wanting to build on an existing open-source project and add new functionality. In fact, that's one of the great things about open-source software! However, if you plan on taking a significant amount of code from an existing project and using it in your own, make sure you follow the license and attribution requirements of the original author(s). Additionally, it may be a good idea to reach out to those authors and let them know what you plan on doing, as they may be able to offer guidance or advice. Good luck with your project!