Place a pillar, go to the next story, and you should see nine transparent tiles surrounding the pillar. Choose a floor tile and place it on the transparent tiles. A ring of transparent tiles will open up around the newly placed tiles, essentially giving you two "rings" of flooring per pillar. Place a pillar four squares away from the first, and two more "rings" will be available. Place as many as necessary to suit your need and you will have a second story supported by pillars.
@ Naga: You can build foundations on a pillar with the cheat "placefriezes on."