Yes you can. Mainly once you beat the elite four, but other more common Pokemon can be found that are not exclusive to the Sinnoh region.
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What you have to do is catch all the Pokemon from all the other regions(complete national dex.
no but if you catch all the hoenn pokemon you can catch jotho and kanto pokemon
Unfortunantly you cant go to other regions in Pokemon diamond and pearl. its not like leafgreen,firered,gold,silver and crystal version where you travel to different regions. You can't, Sinnoh is the only region available in the game.
As in, can you catch them inside the Hoenn Region?
No you cannot go to other regions in Pokemon Pearl or Diamond version but in Pokemon Heart Gold and Soul Silver you can go to the Jotoh region.