You can't get into the hoenn region in Pokemon diamond. It's a 1 region game and that region is SINNOH! BY THE WAY HAVE YOU HEARD OF POKEMON X AND Y
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you can't (get one in the kanto region and it's a key item). but you can make a blue flute in the hoenn region then migrate a Pokemon holding it
If you mean go to hoenn you can't
The Hoenn Region is available in the pokemon versions Ruby, Sapphire, and Emerald.
Hoenn Region
No. You CAN import Pokemon from there, though.