So after the big help of Richard (many Thanks again), I was able to center a loaded model at (0,0,0).
But there is one thing left to do:
When I rotate the model, it still rotates along its original center and not around the stages center. My workaround till now is to move the camera around the model, looking at (0,0,0), but I would like to rotate the model instead.
Is there a possability to set the models center to a different point (0,0,0 for example)?