If you usually have the custom of immersing before Yom Kippur as a spiritual preparation for the day, you may do so even though you are niddah. This immersion does not change your niddah status and is a matter of custom, not a requirement.
You will still immerse on your scheduled night after completing the seven clean days.
Please feel free to get back to us with any further questions.
Gmar Chatima Tova!