Do you mean by hacking or Just playing.
I don't think you can... Sorry... ANYWAY, Unless some one can correct me XD
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I'm Sorry but you cant get more than 18 boxes in Pearl/Diamond
There is no cheat in Pokemon to get additional PC boxes. Those who need additional space in Pokemon X and Y can use Nintendo's Pokemon Bank service.
In each game, there is only one master ball obtainable. However, you can get more master balls with an Action Replay device, or by hacking the game, or by trading over a Pokemon holding a master ball from another game.
You can't!
I believe that Pokemon platinum is better than diamond and pearl because it offers more features, and allows you to do much more cheats and tricks and get more Pokemon through normal game play than Pokemon diamond and pearl. Yes, it is.