Yes, you can. First you must beat the Elite Four (and Champion). Then you must SEE (not catch) all 150 Pokemon in the SINNOH (not national) dex. This can be done by battling all the trainers and legendaries, and by talking to everyone, especially after the game in a place called Celestic town. When you have seen all 150 Sinnoh dex Pokemon, Professor Oak will visit from Kanto and give you a National Dex. Afterwards, a new area called the Pal Park will open up on an area (Route 219). Now, on the continue screen, there should be an option that says "Migrate from xxx". From this, you can choose any 6 Pokemon in boxes to send into your game. This can only be done once every 24 hour period. Then, after you continue your game, you can recatch your Pokemon in the Pal Park (1,000,000 times easier than it sounds).