Well, Atlanta is hardly an international hub (but a huge domestic). So you'll have to do one change somewhere in the world to get to Berlin. Either in the US to get on an AirBerlin flight, or at a major European hub (London Heathrow would be the logical oneworld option). You can also change in Paris (AF), Amsterdam (KLM), Delta fly to Düsseldorf, LH flies to Frankfurt etc.

edit: If choosing Düsseldorf or Frankfurt you can of course take the train to Berlin, but it will be a long journey that I would much recommend to do by plane.