How can i get my crystal pokemon to pokemon leaf green / ruby?

Im currently playing pokemon pearl and have seen a lot of players with pokemon only available in crystal / gold / silver. Ive also seen claims of people getting the whole dex filled. I know the only way to get pokes from my other games is thru pal park. Is there a way to trade from crystal to either leaf green or ruby? Or do i have to get them by cheating? -_-