In the Tools menu, try using Fit Spline. You need to select more than one entity. So if you only have one arc, you will need to split it first.
That makes too much sense, thanks. I had the (in retrospect) silly assumption that arcs would be saved as polylines when I specifically set splines to be saved as polylines.
Thanks, I have it imported now - but is there any way to increase polyline resolution? I guess I can fiddle with my now fully-spline geometry in draftsight and see if anything shows up there.