No.
The only way is through migrating them from Pokémon Ruby, Sapphire or Emerald or by trading.
you can't in diamond without migration, however you can get them in platinum by trading the event lvl 100 regigigas form diamond or pearl to platinum, then they will appear in certain rooms that were once empty. Regice: in mt. coronet on the way to snowpoint city Registeel: at the final room on iron island before you go outside Regirock: unknown. sorry is that for platimun or diamod??
You cant catch them on diamond pearl or platinum only on sapphire,crystal or blue sky and rarely on ranger.ultra rare on ranger.
You can't. You can get it only through migrating, trading or cheating.
get regice regirock and registeel and go to snowpoint temple
You have to first get Regice, Regirock and Registeel either by trading or migrating from a previous GBA game. Once you have all three with you, go to Snowpoint and into the temple that's not far from the Pokemon centre. (They'll let you in if you have beaten the Elite Four) Just make your way through the temple until you find a statue of him. Talk to it then you will battle.
Cheating, hacking, or trading.
No. Migrating is when u put a GBA Pokemon game in the GBA slot and get it in diamond thru the pal park. Trading u get a Pokemon from another trainer.
Regigigas is "sleeping" in the lowest level of Snowpoint Temple. You must have Regice, Regirock, and Registeel in your party to awaken it. These three can be obtained by trading from another person, or by migrating them to the pal park from Ruby, Sapphire, or Emerald.
yes.
In Diamond & Pearl? Impossible without migrating, trading, or Action Replaying.
you get regigigas on diamond and pearl by trading regirock, registeel, and regice from emerald after beating the league then going into the ruin things at snowpoint city, and then finding the statue of regigigas and it should come to life and then you fight him lv1.