I am not shore I understand exactly your word “convert”.
If your spline is 2D it’s easy to follow the vertexes. But you will lose an the aspect. The points used for the construction of the spline are with DXF code 11. So extract them, and draw a pline using those points. Is that what you need? The program is simple, it operate on the last entity from the DWG.
Code:(defun C:spl() (setq pli nil) (setq old (getvar "osmode")) (setvar "osmode" 0) (setq li (entget (entlast))) (foreach at li (if (= 11 (car at)) (setq pli (cons (list (cadr at) (caddr at)) pli))) ) (reverse pli) (command "pline") (foreach at pli (command (list (car at) (cadr at)))) (command "") (setvar "osmode" old) )