Is it possible to gain muscle mass and weight on pancreatitis low fat diet?

Sure: depending on your specific metabolism, gaining weight and muscle mass is just a function of working out, combined with proper nutrition- in a lot of ways a pancreas friendly diet is the same as a good balanced diet that is recommended for diabetics, heart patients, obese patients, etc.go to a certified dietitian they will be able to help design a diet for you to achieve your goals and be healthy

Yes: Even with diet low in fat, it can be high in protein and this would allow you to put on enough weight that can turn into muscle mass.