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.
without hacking, 100 with hacking, 999 and then reset to 0 and no more exp gain.
There is no such code.
I'm Sorry but you cant get more than 18 boxes in Pearl/Diamond
yes, i do know the code if want to now go to www.pokemonactionreplay.org/pokemon+diamond/action replay.net.com.poke
There is NO WAY up to date. But soon Nintendo will try to make something like it with transferring. (Example: Pokemon Ruby and Shappire Box (Gamecube) )
No you can not have more than one login in Pokemon diamond.
Hacking
If you mean shaymin, then you need to get a DS action replay and find the code that lets you go through with that extra shymin event. Its not cheating or hacking because they made the DS replay just so they could get more money from Pokemon fans wanting to play a couple more events in their diamond and pearl games.
Your boxes increase after you max out the first 8.
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.
Arceus was never released anywhere legitimately, so you can only get it by trading with someone, or by hacking. Arceus is obtained through event's which are quite done now but more will come soon. You have to trade it into soul silver from Pokemon pearl, diamond and platinum.
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.