you can put a new brood box under. i do it most of the time. it usually works better for me than putting one on top. if you think about it, it's the way hives are natural built. comb is built in a downward direction. honey is stored across the top and down the sides, usually in the upper half of the hive.
i have had hives that don't want to build down, but more that don't want to build up. in either case, either move brood into the new box, or if that fails, swap the boxes. moving brood into a new box has become standard for me. 9 out of 10 times it works.
if you are running 8 frame mediums you'll need 3 or 4 boxes for brood. you should have good stores in by now. i agree with brian about feeding if there is any kind of flow going on in your area. if not, yes, you may need to feed to support the brood and build stores.