hyphenate-backwards

(require 'cl-lib)

(defun hyphenate-backwards (n)
  "Insert hyphens between words N-times (prefix arg) behind point."
  (interactive "p")
  (save-excursion
    (dotimes (_ n)
      (cl-loop do (backward-word)
               do (backward-char)
               until (looking-at " "))
      (delete-char 1)
      (insert "-"))))

(define-key text-mode-map (kbd "C-,") #'hyphenate-backwards)