Haha, most of the kids on CC aren't really the athletic type, if you haven't noticed already. =p
If you have entered competitions, then that's great - you have something to solidfy your ECs with. Put down any medals or places you've gotten in competitions, or any other awards you've gotten related to the sport.
If you haven't really entered any kind of competition, then it's a bit tougher - but you can make it work. If you want you can simply put down "skateboarding" or "BMX" as an EC, but it won't be too compelling of an EC. If you can kind some kind of meaningful activity that can relate to the activity, then that will really work in your favor. For example, instead of just "skateboarding", you can do something like "teach kids about skateboarding" or "volunteer at a skate park" or "run a skate shop".
So you can simply put down "skateboarding" if you're pressed for ECs, but it will look a lot cooler if you can turn your passion into something more significant.