You don't have to modify your tank, it be helpful and aesthetically pleasing, but you can purchase an overflow box that hangs on like a hang on the back filter that transfers the water down to your sump.
There are various set ups, for beginners a basic tank that you can place your skimmer, reactors, etc and can serve as a fuge and/or increase water volume.
What size tank do you have now? And does your stand allow for sump tank under?