Is it possible to import AutoCad DWG models as edges and nodes?
No, not directly. It is very well possible to use AutoCad DWG models to build a Visum model from. There may be two different cases: 1) The DWG file contains a CAD model showing streets, crossings, pavements, cycle ways, lanes and lamp posts in all detail. 2) The DWG file contains a CAD model holding a nodes-edges-model.
Ad 1) These models are not suitable as Visum model, though as background very suitable. Having the detailed street as image in the background, a nodes-edges model is built within Visum more easily. To use such DWG files as background, you can load them in Visum as background file of type DWG.
Ad 2) These models might be suitable in many cases as foundation for a Visum model. However, Visum cannot read DWG nor DXF directly as nodes and edges. Therefore you need a detour. A possible detour leads over the DXF format, which will be transformed into the SHP format (Esri-Shape). Visum can read SHP files as edges and nodes.
You might find suitable tools in the web by giving in DXF2SHP or use a GIS program like MapInfo.
Remark: Visum 9 offered the old program DxfKonv.exe (typically in c:\Programme\PTV_Vision\VISUM940\DXF\DxfKonv.exe) but this only transforms DXF files into backgroundfiles (according to the old format HGT).