Not necessarily. For example, if you are committed to a certain class one day and want to swap out the session in Cut. for that, go ahead. Or, If you’re just looking for some new workout ideas then feel free to incorporate those into your own routine (like the HIIT sessions for example!)
Saying that, I really wouldn’t recommend adding extra sessions if you’re following the programme if you want the results to work because it’s all been thought out in a way where the rest days and moves allow you to recover optimally before hitting them hard again. The sessions are intense, and the week is about having 4 highly intense sessions - adding in training on top of what’s here is likely to slow down results and creates a risk of injury or burn-out.