Someone on GoogleGroups linked me to the answer: https://groups.google.com/group/comp.lang.lisp/msg/7e24e8417cd1b6e6?dmode=source
(defun direct-slot-defn->initarg (slot-defn)
(list :name (slot-definition-name slot-defn)
:readers (slot-definition-readers slot-defn)
:writers (slot-definition-writers slot-defn)
:initform (slot-definition-initform slot-defn)
:initargs (slot-definition-initargs slot-defn)
:initfunction (slot-definition-initfunction slot-defn)))
(defun add-slot-to-class (class name &key (initform nil)
accessors readers writers
initargs
(initfunction (constantly nil)))
(check-type class symbol)
(let ((new-slots (list (list :name name
:readers (union accessors readers)
:writers (union writers
(mapcar #'(lambda (x)
(list 'setf
x))
accessors)
:test #'equal)
:initform initform
:initargs initargs
:initfunction initfunction))))
(dolist (slot-defn (class-direct-slots (find-class class)))
(push (direct-slot-defn->initarg slot-defn)
new-slots))
(ensure-class class :direct-slots new-slots)))