Just thinking about the various weapon prefixes, it's generally clear that most of them can't be added later. You can't make your bow into an Elven bow later on. Your mace can't become Dwarven through some add-on. It's way to late to make a weapon Fine.
Balanced is a little tricky. Changing the balance of a weapon is possible. I'm not saying trivial, but changing a hilt, modifying the shaft, adding or changing the weight of a secondary point, etc. would probably work for many weapons, given a properly made weapon in the first place.
I'm thinking it's not going to be cheap - not as cheap as one that's been properly balanced in the first place. Probably changing +6 CF instead of +4 CF as a retrofit would work.
I'm going to put that in front of the group and see if it flies.