(defun my-insert-tab-in-head-of-lines-on-region (begin end)
"選択した領域のすべての行頭にTABを入れる"
(interactive "r")
(goto-char (max begin end))
(let ((begin (min begin end))
(end (point-marker)))
(goto-char begin)
(while (<= (point) end)
(when (bolp) (insert "\t"))
(forward-line 1))
(set-marker end nil)))
とか。