If you want to increase the fiber content for your cat, I recommend to start with the food. Over the counter weight loss foods tend to have higher fiber contents. There are also many prescription foods available through your veterinarian that have a higher fiber content.
If you want to add fiber to treat constipation, Metamucil or psyllium can be used short term. Long term use of these supplements is not recommended as they can cause dehydration and make constipation worse over time.