Hi Jens !
I move the thread.
You can use dxf to bring the street network over, make sure the data is actually curves and not polygons (for streets). There's only linear segments supported atm, so any true curves need be segmented into some linear curves. Though don't segment them too dense, this may cause issues. Rather, in CE, use the graph cleanup tool and then set the street curvature again in CityEngine.