(defun C:MPPL

; = Match Properties of Polyline to Line [including converting Line to Polyline]
  (/ pl lin)
  (setq pea (getvar 'peditaccept))
  (setvar 'peditaccept 1)
  (setq
    pl (car (entsel "\nSelect Polyline to match Line to: "))
    lin (car (entsel "\nSelect Line to convert to Polyline and match selected one: "))
  ); setq
  (command
    "_.pedit" lin ""
    "_.matchprop" pl lin ""
  ); command
  (setvar 'peditaccept pea)
); defun

 

It ought to be enhanced with valid-object-type selection controls, maybe command-echo suppression, error handling, and all the other usual bells and whistles, but as a start it works for me, in limited testing.

I've managed to juggle your code with the addition of entlast in

the matchprop call instead of lin, which holds only the original data

fo the selected line, and not the new polyline.