You don't get any Pokeballs.
So no, you can't.
You get some Poke and Heal Balls as a reward for finishing the game for the 3rd and 6th times, but those dont go to your bag, you can only transfer them to the full ORAS games.
PS. You do catch Glalie/Steelix, but that's a part of the "story" of the demo.
Source - played the demo.