What you want to do is this:
(defun listify(ls)
(mapcar (lambda (elem) (list elem)) ls))
EDIT
Which is the same as (Thanks to @RainerJoswig):
(defun listify(ls)
(mapcar #'list ls))
And if you do:
(listify (list 1 2 3))
or
(listify '(1 2 3))
The output will be:
((1) (2) (3))