I know it's an old post, but I actually found it trying to solve the same problem, that is, merging .__a .__b, ..., files, and since I managed to solve it, I thought I would post what I did here, for the next one :)
I've tried many Mac software and none of them worked. So I download a free trial of Crossover, start a new XP Bottle, downloaded FFSJ and launched it with this bottle. That worked just fine, was perfectly legal and free, and took about 10 minutes (downloading included).
Hope that can help!