Growing up in L.A. we never had a problem with our brown sugar drying up. But here in Pasadena, I guess the climate is just so dry, brown sugar doesn't last very long, inside or outside the 'frige.
But I experimented the other day and found out you can indeed remoisturize hardened brown sugar by steaming it. I lined my steamer with a cotton bandana so none of the sugar would fall through the holes, but I'm sure there are other methods. I suppose one could place a bowl of hardend brown sugar in a pot, pour some water around the sides, cover it, turn on the heat, and steam it that way.
Or you could buy a new package of brown sugar for 89 cents. But that's no fun.