Its likely composed of AutoCAD’s equivilent of polygons, called faces or regions (maybe… don’t know the terminology for sure). However, I do know they’re pretty useless to work with in ACAD. You’ll be much better off either recreating the entire model in ACAD or doing your desired functions back in Blender.
For adding a chamfer to specific edges, look for the bevel-center script in the Python Scripts -> Mesh menu. This should let you perform some limited and clumsy (compared to CAD) beveling. As to the “other work” you’re looking to perform (maybe punching holes, or somesuch?) there are likely ways to do these with mesh modelling, but you have to be creative in how you go about getting them done. Fortunately, blender is flexible enough to allow a lot of such creativity. There’s probably a tute out there somewhere for what your looking to do. The fun part’s finding it!