Trading Pokemon

Yes, there is actually a way that you can trade from the Third Generation games on the Gameboy Advanced such as Pokemon Fire Red, to the new Fourth Generation games on the Nintendo DS, which are Pokemon Diamond, and Pokemon Pearl. This method of trading is called Migration. Now in order to migrate Pokemon you have defeated the Elite Four, and got the National Pokedex from Professor Rowan by seeing all 150 Pokemon in the Sinnoh Region.

After you do all that, head to the Pal Park, you can find where it is at by looking on your map. Go there, and then save and turn off your game. Insert Pokemon Fire Red into the GBA slot on your Nintendo DS, and turn it on. Find the six Pokemon that you want to Migrate to Diamond, remove any HM moves that they know via move deleter in Fuschia City and put them in the PC. Save and turn off Fire Red. Turn on Pokemon Diamond and an the main menu click Migrate. Choose the six Pokemon then start the game, go to the Pal park and talk to the construction worker so you can begin the trade.

Hope this helps.

Will why don't you mirgrate. Or use action replay.

Yes AND No If You "Trade" From Leaf Green OR Fire Red The Pokemon Cannot Be Returned Although It Is Not Called Trading It Is Migrating...
I Hope This Helped You On Your Quest To Complete The National Pokedex...

Beat the game and then go to Pal Park and save. Then put your choice of gameboy game in and start Diamond or Pearl. Then click on migrate from *********. Then select your pokemon. Start and go to the guy beside the counter in the building. Go in and catch all the pokemon. They will never pop out so that aids in your quest. No other Pokemon pop up which means you have to go in the exact patch of grass which is kind of annoying. *HINT* Once you've been to Pal Park you can fly there. Go to the bottom of the map and it should say something like route 205.