So this sounds like if I can write the coordinates of a bock in AutoCAD to an XML file, I can use an interface in SketchUp or Rhino to import those coordinates and swap out that AutoCAD block for a SketchUp component?
Sounds simple in theory, I need to figure out where I can learn how to write XML files.
Yes, you can even hold the modelling data of the 3d object in XML as points, edges, faces and manifolds along with other no graphical attributes.
There are 2 ways to approach this, one would be to create your own XML schema for your files or use a well-known schema such as IFC or LandXML. If you need to store graphical data I'd lean towards the latter and you may be able to find tools to use at both ends of the CAD systems already available (i.e. that can read/write IFC in AutoCAD or Rhino say).
If you are only using placement and non-graphical data from AutoCAD and you have 'blocks' in the other CAD system then a simple XML schema of your own would suffice.