National pokedex

Yes, you can get all 493 Pokemon without using an Action Replay, but you would have to have all the Pokemon games such as Pokemon Emerald, Poke Fire Red, Pokemon Leaf Green, Pokemon Sapphire, and Pokemon Ruby, so you cuold trade the Pokemon from those games to Pokemon Diamond, or Pokemon Pearl. It can be done, but it does take a lot longer than it should.

Well of course it's possible, but the process is very straining. In order to accomplish so without cheating, you must have all the Gameboy Advanced Pokemon Games (Ruby, Sapphire, Emerald, FireRed, and LeafGreen), and catch all the Pokemon in each game, and then what you have to do is migrate 6 Pokemon from each game once a day into Diamond/Pearl.

Then, you need to catch all the pokmon in the Sinnoh Pokedex in Diamond/Pearl, which requires trading from the opposite of the game. What I mean is, in order to get all the Pokemon in Diamond, you may need to trade from Pearl, and vice versa.

With that said, you'll need luck D:

You would also need to get Pokemon Ranger, in order to obtain Manaphy and Phione. You get Manaphy after doing the Manaphy mission on Pokemon Ranger and then you use two dual screens and you put Ranger in one and Diamond/Pearl in the other. Then go to any Pokemon mart and talk to the man in green. To get phione, you have to breed your manaphy with a ditto. NOTE: Phione doesn't evolve into Manaphy. Also to get Lugia you can go to an event or get it from XD. To get Jirachi, Deoxys, HO-OH, Darkrai, Arceus, and Shamin you need to go to a Nintendo event(none so far held in the United States, only in Japan)