UML: Modelling run-time method adding

Hi all,

as far as I know it is possible with Ruby to add methods to objects at
run-time which the class of that object didn’t define.

Does anyone of you happen to know how an object and a sequence diagram
(showing the process of adding a method) in UML can be drawn in order to
indicate that a method is/has been added to an object?

Thanks in advance and kind regards
Markus