Yes. Any existing project (e.g. a standalone application, Twitch Extension, chatbot) that adds Channel Points functionality for a hackathon submission is eligible. Review of the submission by the judges would mostly focus on the new functionality built for the hackathon and how it enhances your existing product as a whole, so please make sure that your submission description is clear as to what was pre-existing, what was added for the competition, and how it improves the application for the viewer/broadcaster experience.
There was a similar question regarding reusing existing code here if this further detail is also helpful. If either of these answers lead to more question, let us know in the comments :-)