If its an internal overflow you can do a durso design. It is a a U shaped top with a hole drilled in the top to let air through. You can google diy durso to see examples.
if it's a hole drilled through the side you can hard plumb something similar on the outside. Depending on how far the hole is drilled from the top of the tank. You would have elbows brining the pipe slightly above the level of the hole (outside the tank) then out to a T. The T would be vertical with the bottom heading to the sump. The top of the tee would have an extension above the top of the tank with a cap on it with a smaller hole ( start small and adjust the hole to proper size. if its too small it will siphon (it will sound like a toilet flushing every thirty seconds, too big and you will start to get overflow noise) drilled to allow air flow so that you would have a constant flow of water instead of a surge siphon scenario. I wish I could draw this to explain it better, but basically it is an external durso, the top vent pipe would be well above where the water line will ever be so no worry of leaks.
Hope this helps, let me know if I can eplain this better.
