Do I need a external flash to take sports pictures under lights on the field at night? I'm using RAW and manual exposure.
In general, no, not only do you not need it, but it won't help at all. In order to provide any meaningful amount of light more than a few feet away, you need a very, very powerful flash — much more than you can get in anything battery powered.
Unless you are right on the field — less than ten feet away from your subject — even an expensive hotshoe flash will not do much. If you're a little farther than that — twenty feet, maybe — it could help a little bit if you also have the ISO cranked up pretty high. But mostly, it's just a waste. Light inherently diminishes with the square of the distance, which means that every bit further you want to reach takes a lot more power, and there's no magic solution there, unfortunately.