As long as the tank is not tempared you can drill it, install a bulkhead, and and skimmer, however you would need an empty tank for that.
Over flow box: (from madisonreef.com)
Drilled tank with custom surfice skimmer (from madisonreef.com)
You can make your own overflow box very easlily and for only about 20 bucks depending on what you have laying around. Otherwise you can buy one for 50 to 150 dollars. This one is the cheapest i can find.
Eshopps Overflow Boxes
Otherwise like i said you can also drill your tank. All you need is to make sure you don't have a tank that is tempared glass. The drill bit you use has to be a dimond hole saw. In that video it shows how he drills the sump. As for the surfuce skimmer all you need is glass or aclric cut and then silconed into place. Normally you place it a quarter of an inch under where you want your water line to be.
Hope this helps a bit in your project.