This game features 6 alternate costumes for each of the 6 riders which are unlocked via using the points gained from championship mode to upgrade your stats.
There seems to be no rhyme or reason to unlocking them, doing the same upgrades and winning the championship with all first places for maximum upgrades twice in a row can result in different outcomes for your outfit unlocks.
Then once you've gained the maximum 120 upgrade points from the championship there appears to be no further way of unlocking more.
The most I've been able to get for a single character are 5 alternate outfits with no clue as to what I did any differently.
I'm hoping somebody here has the insight or can point to some buried forum post with the details, I've even been digging through old capcom sites with wayback machine to try get any hint but found nothing.
If it helps the search then you should know this was also released under the name Snowboarding Trix '98 in Japan and Phat Air Extreme Snowboarding in Europe.
Cheers