coolant flush, 3800 series 2 pontiac enigine, radiator cap can not be removed and i need to know how to add coolant

The main fill happens via the radiator. The cap is removable and must come off if you're going to refill the system. The final filing occurs at the plastic overflow/expansion tank. There is a bleeder valve near the thermostat that you will use to try to get the air out of the system. That is the tough part and is absolutely critical to avoid overheating and engine damage. Apparently filling the system with the car facing uphill or the front wheels up on ramps makes the process easier although I haven't actually tried it.