^I think he means trading Kanto pokemon that are on his emerald to his ruby, meaning that they have already been transfered over from FR/LG
Yes, you can, if your emerald has the national pokedex, which I suppose you do have since you need that to have Kanto pokemon that aren't part of the Hoenn dex. You only have to make it up to Rustboro City in Ruby, since I think that's when you're allowed to trade.