If you have added something original, non-obvious, and useful that improves the original invention, you may be able to obtain an improvement patent. But, beware: an improvement patent will not give you the right to manufacture and sell your improvement. To do that, you will need to obtain a license from the original patent holder (unless that patent has expired). However, it will prevent the original patent holder from manufacturing or selling your improvement unless it obtains a license from you. Often, the holders of the original and improvement patents will cross-license so each may manufacture and sell both the original and the improved invention. Patents are a very complex subject and you should not attempt to operate in this area without the advice of a licensed patent attorney.