You can't really train JUST a broad sword.
Yet, in RM you can. That is just one of the foibles of a skill-based system, if a player forgets to do something, then they don't get whatever it is they forgot. In this case, a weapon skill where it makes no sense for them to not have picked up.
But, for me, that is just one thing a GM has to deal with and help the players out when they forget, and just let them do a little shuffling of points. (Heck, they aren't actually there in the world with their life on the line, so forgetting is understandable.)