(defmacro add-key-value (key value alist)
  `(push (cons ,key ,value) ,alist))

(defparameter *a* nil)           ; => NIL
(add-key-value "foo" "bar" *a*)  ; => (("foo" . "bar"))
(add-key-value "baz" "qux" *a*)  ; => (("baz" . "qux") ("foo" . "bar"))