There are several ways an auto-topoff system can be added to a tank that are all dependant on budget.
For a few bucks, you could DIY a gravity-drip using a container, where an IV line is siliconed to a container (5 gallon bucket, old milk jug, etc.) and topoff water flow is regulated via a pinch valve on the IV line. Kent manufacuters a product like this called the Aquadoser. This method is also a great and inexpensive way to add Kalkwasser to your system. Gravity dripping has a few disadvantages though; the line can during kalk dripping if not properly maintained, and matching the drip rate to evaporation gets tricky, especially as the evaporation fluctuates throughout the year.
With a slightly larger investment, you could purchase a float switch for your future sump. The switch mounts in the sump (as that's where your water level will fluctuate) and is connected to a powerhead in a resevoir or a dosing pump that pulls from the resevoir. A dosing pump is recommended because a powerhead could easily flood your system if the float switch goes bad, where a dosing pump is only capable of adding a few gallons a day, and would give you much more of an opportunity to avoid disaster. In fact, a dosing pump could be used on it's own, but you would have to program it to keep up with evaporation.
Finally, there are some all in one kits on the market that work great. Reef Fanatic makes a water level controller that utilizes two sensors (with one being used as a failsafe). Tunze makes the Osmolator which uses an infrared sensor mounted in the pump to control a small pump in the topoff resevoir. It also has a few failsafes built-in like a float switch to shut the system off if the sump level gets too high, and the controller will automatically turn the pump off if it's been running for more than 10 minutes. I recently added one of these to my tank and it's been fantastic.
