You could change the *debugger-hook* variable to handle the conditions as what you want, e.g.

(defvar *use-slime-debugger* t)

(setq *debugger-hook*
               (let* ((slime-debugger-hook *debugger-hook*)
                      (new-debugger-hook  (lambda (&rest args)                                                        
                                            (if *use-slime-debugger* 
                                                (apply slime-debugger-hook args)

                                                ; do what you want

                                                ))))
                 new-debugger-hook))

But be aware, there might be additional problems you could meet.