If you want to gain Hoenn Pokemon on Diamond, you will need a regular DS or DS lite, with the GameBoy slot.
At the ingame start menu, there will be an option to "transfer Pokemon from game". but it will only appear if you have already defeated the Pokemon league. After you have beat them, then the Pal Park at the southern area of the map will be open. Go inside of the Park building, talk to the man at the counter and your good to go! Hope this helped you :D
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No because it is a sinnoh Pokemon not a kanto, johto, or hoenn
If you mean go to hoenn you can't
Turtwig does not appear in Pokemon Sapphire, or in Hoenn for that matter. Instead, you can find Turtwig in Generation IV (Diamond, Pearl, or Platinum).
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
you can't catch any hoenn Pokemon in firered or leafgreen you must trade hoenn Pokemon from ruby or sapphire or emerald to leafgreen or firered.