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

(defmacro add-key-value2 (key value alist)
  `(setf ,alist (acons ,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"))

(defparameter *b* nil)            ; => NIL
(add-key-value2 "foo" "bar" *b*)  ; => (("foo" . "bar"))
(add-key-value2 "baz" "qux" *b*)  ; => (("baz" . "qux") ("foo" . "bar"))