you cant you can only transfer from gameboy games to ds games and they don't go back down(new preson)pla,pla,pla!don't listen to this guy.you can. have both games in ds.go to pkmn pat the main...
When you have completed the Pokemon league and completed the sinnoh dex then you will unlock an area called palpark then save and turn off the ds, insert sapphire in the gba slot of the ds turn on...
You can't transfer any Pokemon that does know moves like Cut or that from GBA , but you can get anyother Pokemon from it. You just can't send them back. Choose them wisely.