Yes you can. Don't believe anyone why says that you can't because I know for a fact that you can.
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he will battle you in the kanto region but you need to have gotten the enigma stone to catch latias/latios he'll battle you in pewter city or saffron city
After you get your Hoenn starter and speak to Steven in the Pewter Museum, Steven will be in the Silph Co. bottom floor. He has a Beldum with a Dawn Stone to trade for a Forretress.
Steven Stone is the most powerful trainer in Pokémon Emerald. He can be found in the upper area of Meteor Falls after defeating the Elite Four.
its a wifi event.
You need to get latias ( can only catch it on Pokemon heartgold) and latios (can only get it on Pokemon soulsilver) and speak to steven who is in the silph co. in saffron city.You must get the Enigma Stone from the WiFi event, which turns out to be the Soul Dew.