There's no easy way, but I'd do it one of two ways.
Trace it with polyline arcs. By typing "a" during the polyline command, you can trace it with a series of tangent arcs by snapping to the existing lines.
I'd probably do it this way, though. Trace it with splines, snapping to a few existing points, and then carefully moving the points of the spline to fine tune the shape. Then use a spline to polyline conversion tool, or save as v12 .dxf to convert to ploylines. The conversion tools give you more control. You'll still have the shape be made up of straight segments, but it should be much smoother.