This almost sounds too simplistic, so I may be off base with what you're attempting but a 2D polyline is no different than a 3D polyline until you determine a "Z" elevation at some point along your polyline.
Draw your polyline and do a "LIST" on it. It should return a "Z" at 0,0. This should be the same whether your command was PLINE or 3DPOLY.
With a 3dpoly one can have different elevations at each point.
I have quite a few drawings in 2d that I need to convert over to 3d for a cnc system.
I wrote a lisp routine to read the 3dpoly points and store the x,y,z, coordinates at each point into a file.
I want to get a ways from running this on a 2d polyline and hand editing the text file, putting the z values in. If I can automate the process I could elimates some errors.
