Can I add more exercises to the Athlete 20XX workouts?
If you want to do more core work or target different parts of the body (like forearms and biceps etc.), this is fine to do as long as it's done after the Athlete 20XX workouts.
Adding isolation exercises can be beneficial but it is not 100% needed unless you are an advanced athlete. If you have clear weaknesses or imbalances, then adding specific exercises is a good idea.