Thibaut, yes it works now!
Haddad, I could test some things if you want. In my library works, I normally work (inside OM) with string inside a string (I am not sure if it is the correct name). It works very well.
(let* (
(action1 
(om-cmd-line 
 (string+ 
  (list->string-fun (list (namestring "MY.EXE path")
    " --input " (list->string-fun (list (namestring (file-pathname sound))))
    " --output " (list->string-fun (list (namestring (outfile sound-out))))
    " --plugin " (list->string-fun (list (namestring (define-VST2-plugin plugin-path)))) "," 
                 (list->string-fun (list (namestring (define-fxp-presets fxp-path)))))))))
(ckn-clear-temp-files)
(make-value-from-model 'sound (outfile sound-out) nil)))
The list->string-fun it is this code:
(defun list->string (ckn-list)
  (when ckn-list
    (concatenate 'string 
                 (write-to-string (car ckn-list)) (list->string (cdr ckn-list)))))
                
; Code by "https://gist.github.c/tompurl/5174818"
Thank both of you for the help!