@Thomas103 , welcome to the Community.
It is very difficult to add more fans in small cases. You will have to become very creative and make certain that there is a fan header to use. Here is a video that will explain Case Pressure:
It is very important to assure that you are not making more problems by adding a fan in the wrong place. I believe that you would most likely have to add a fan that is no larger than 90 mm (3.54331"). You should measure the existing fan. The fan header, if there is one, will determine the number of pins.
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I am not an HP Employee!!
Intelligence is God given. Wisdom is the sum of our mistakes!!