Illustrator creates those curves using spline method (control points, vertex direction, etc..) so that's how AutoCAD importing them. But you can google a spline to pline lisp and do the conversion yourself.
What do you want the imported models from SketchUp to be?
I'm not sure about the sketchup issue. The designer uses it because it's so fast to illustrate a simple design but it doesn't translate into CAD very well. I, as a rule, redraw his files anyway due to the lack of precision in Sketchup. I cannot use his 3D models because, whereas it is a block reference, it contains a lot of geometry within it and tends to bog my machine down. Exploding it reverts it into faces and face feceted curves that require a redraw anyway.
I didn't know if anyone had come across this as well and found a simple solution to using the files.