Trading in pokemon help?

i <3 pokemon!!!! i just beat the elite 4 in black..... ok so back to the question.... you cant trade pokemon to the white/black until you have the national pokedex(beat the elite 4) then when you beat them... you go to this place.... the bridge right when you exit nimbasa city.... to the marvelous bridge. sadly you cant take your pokemon back once they enter the white/black game TT-TT