Example, how to patch 3party library on fly, converting a ordinal function into a generic.

convert-3party-function-into-generic.lisp

(setf (fdefinition 'original-report-expected-line)
      #'prove.reporter.list::report-expected-line)

(defgeneric report-expected-line (report)
  (:documentation "Returns a line which describes a problem with failed test."))

(defmethod report-expected-line (report)
  "Default behaviour is to call original Prove's function,
which formats a string, comparing two values."
  (original-report-expected-line report))

(setf (fdefinition 'prove.reporter.list::report-expected-line)
      #'report-expected-line)

(defclass failed-assertion-report (prove.report:failed-test-report)
  ())

(defmethod report-expected-line ((report failed-assertion-report))
  "Blah bla bla bla")

(prove.reporter.list::report-expected-line
 (make-instance 'failed-assertion-report
                :got t
                :expected t)) => "Blah bla bla bla"

Thanks to beach, from #lisp channel on freenode.