These are all the cheats I know for this game, all of these work, I know because I have tried them.
These cheats you can unlock the characters if you play through conquer the mountain but there's a few extras too I think: slicksuit-Hiro wheresyourtail-Bunny San boneyardreject-Gutless milkemdaisy-Cudmore the cow tankengine-Churchill greatwhitenorth-Canhuck worm-eddie(from ssx tricky) bronco-luther(from SSX tricky) notsosvelte-Svelte Luther zenmaster-Brodi skin brokenleg-Jurgen(from ssx) back2future-Marty(from ssx and ssx tricky) finallymadeitin-Unknown rider callhimgeorge-North West Legend Windmilldunk-Stretch
These will unlock all extras: djsuperstar-All music postnobills-All posters nogluerequired-All toys gotitgotitneedit-All trading cards graphicdelight-All boards(plus a few extra ones!) myeyesaredim-All videos naturalconcept-All artwork(if you ever see it spelt koncept it is incorrect because i have tried it twice, it doesn't work!!) shoppingspree-All peak 1 clothes
Another cheat is midstationexpress but I am not sure what it does but it does work.