Yes, download the PKM file from here:
http://db.pokesav.org/main/?q=Event-Downloads-DP-USA
Then open PokeSav, click Storage, and in one of the empty spaces, click load then open that PKM file. When you have it in your party you can use it to catch the Gen 3 Regis, then use those to catch your own legit Lv 1 Regigigas. Before you do, I recommend you have a Smeargle with this or a similar moveset:
Spore/Hypnosis/Sleep Powder/Grass Whistle (Spore is best because it works 100% of the time vs 50-75%)
False Swipe (Get it down to 1 HP)
Mean Look (later for like Cresselia etc)
Recover, Softboiled, Milk Drink, etc. (Or whatever move you want)
That Smeargle will make it easy to catch any Pokemon, legendary, running legendary, or even regular Pokemon.