There are different types of patents available that serve different purposes. The two most common types are utility patents and design patents.
A utility patent protects the invention of a new and useful machine or a new and useful improvement thereof and allows the patent owner to prevent others from making, using, or selling the invention for a period of time while the patent is in effect.
A design patent protects the new, original ornamental design that is applied to an article.
Based on what you have described, it appears you would be looking to file a utility patent to protect your add-on.
If you would like to consult with an attorney about your idea and determine what an appropriate course of action would be,can connect you with a skilled patent attorney from our vetted pool of legal professionals. He or she will be able to provide you with a free 20 minute consultation to discuss your idea and determine how he or she can assist you.