You can edit ActiveAdmin controller actions, but if the action you do after update is the same when updating form outside admin panel than it's better to use callbacks I guess. Why writing more code?
http://activeadmin.info/docs/8-custom-actions.html#modify_the_controller
ActiveAdmin.register Post do
controller do
# This code is evaluated within the controller class
def define_a_method
# Instance method
end
end
end