You dont need to drain and drill it, you can get an overflow box to get water down to sump. But they also make awesome HOB skimmers, and even a few that go directly in the tank. I would check out the reef octopus skimmers or the hydro bubble skimmers. Be sure to get one that is rated for twice your water volume and be wary of cheap skimmers.
But its important to realize that your rock is your primary source of filtration in your tank, which why its important to not skimp on the rocks. You want around 1-2 lbs per gallon. The other stuff is just to help out and make it a little easier to keep good water quality.