First of all, in order to freeze an action, you need to use a fast shutter speed setting. With fast shutter speed, less light is allow to enter the camera. If it is a outdoor events, usually it is not going to be a big problem because there are plenty of light. The problem is with indoor events.
Lightning inside the stadium usually is not as bright as outdoor. In order to take a proper exposed photos with your camera, you need to set a wider aperture (lower aperture number in your camera) so that it allow more light to go in.
The maximum aperture of your setup depends on the lens you use with your camera. If the lens you current cannot set to a lower number such as F/2.8, F/1.8, F/1.4 or F/1.2 ... then you may need to get a different lens. The wide aperture lens is usually refereed as fast lens.
Beside using a wide aperture lens, change the ISO settings to a higher number may also help.
As for the flash, you will need to very powerful one, I doubt the build-in camera flash can do the job.