Вы можете добавить маршрут по умолчанию:
routes.MapRoute(
"Topics",
"Topic/{topicName}/{action}/{topicId}",
new { controller = "Topic", action = "AddQuestion", topicId = "1" }
);
routes.MapRoute(
"Default",
"{controller}/{action}/{topicId}",
new { controller = "Topic", action = "AddQuestion", topicId = "1" }
);