(Original code source)
Depending on where you want the label to appear, change:
(setq str (car line))
to:
(setq str (strcat (car line) ",Setout"))
This will result in:
number,Setout,x,y,z
If you want it at the start, change it to:
(setq str (strcat "Setout," (car line)))
to yield:
Setout,number,x,y,z
If you want it at the end, change:
(write-line str file)
to:
(write-line (strcat str ",Setout") file)
which will yield:
number,x,y,z,Setout