There is no way I know of to increase the reels drag and I can't think of a good reason to do so. If what you want is a heavier trigger for when you use large bait so they don't keep popping the flag then you can file the trigger slot deeper. I use the HT Polar Therm Extremes that are essentially the same disc and mechanism. Mine have a light setting and a heavy setting on the trigger and I did file the heavy to be deeper and solved the bait flag problem.
Gary