You can use luggage forwarding (takkyubin) service. Upon landing at the airport, send your equipment to the ski resort. Allow next day delivery.
Of course, you could just choose to rent skis. There will be skis available to rent at the resorts. If you are particular about your equipment, there are usually ski shops near the slope that rent higher-end stuff.
Plenty of skiers and snowboarders bring their own gears to Niseko. The direct ski buses from Chitose airport have large trunks to store luggage and gears.
To some extent it can depend on your type of skis and also airline ski baggage costs. Renting may be a good opportunity to try some wider powder skis (which can be changed daily). You could still bring your own boots and ski wear.Edited: 22 May 2018, 20:46