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
There are 8: Kanto, Johto, Hoenn, Sinnoh, Unova, Almia, Poketopia, and Oblivia. eerrr... u forgot a couple of regions... there is also the Orre region(which is not shown in the anime but is the setting of Pokemon colosseum and Pokemon XD:gale of darkness) and fiore( a region mage up of four parts). i know you alredy metion this but I'll put it into a little more detail... Almia is the Pokemon rangers headquarters....... Oblivia is made of three major islands, two are connected by a bridge, and there are also several smaller islands, two of them are volcanic.......... and you know the rest...(kanto, jotho, hoenn, sinnoh, unova)... oh and since the unova region is new some people do't know about is... it is the new region that satoshi(ash) takes his new journey with all new Pokemon!
Hoenn Region
there is 160 pokemon
The Hoenn Region is available in the pokemon versions Ruby, Sapphire, and Emerald.
Fiore
no i finished the game
Yep you take a submerine from up the waterfall
Fiore
You get it from Pokemon Ranger, from Fiore or Almia, in the form of an egg.
follow the story line.
unknown
Complete Mission 9
You get It from the Pokemon Ranger from Fiore,Oblivia or the other one that I forgot
fiore temple during a mission after the credits
He goes to the Fiore region and works in summerland
I don't think you can, unless you use a cheat.