Yes, the Revit API provides access to line-based elements such as walls and ducts etc., for instance via the location line property.
AutoCAD and other CAD import does not count as line-based elements, though.
The Revit API does not provide access to all the imported CAD data.
If you are new to the Revit API, I suggest the following:
If you are new to the Revit API, I would strongly suggest that you work through the Revit API getting started material first of all, especially the DevTV and My First Revit Plugin video tutorials:
http://thebuildingcoder.typepad.com/blog/about-the-author.html#2
If you want to dive in deeper, go through the ADN Revit API Training Labs after that.
Then start implementing your own add-ins.
Before looking at the Revit API at all, you definitely need some understanding of Revit from an end user point of view.
It provides a huge amount of complex functionality right out of the box.
If you start programming Revit with insufficient understanding of the basic Revit end user functionality and the optimal workflows and best practices to make use of it efficiently is doomed for disaster.
Take heed, have fun, and good luck!
Cheers,
Jeremy