This thread does a pretty good job of explaining some of the things necessary to join a TS to AD. To be honest, joining ANY legacy Linux NAS to ANY for of AD is a pain in the rear, but JoshC's explination seems as sound as any I've seen over the years.
I may not work for Buffalo, but I've been a technician specialising in routers and Linux administration for just under ten years. I don't speak for Buffalo, or any of their employees, nor are the ideas, thoughts, etc related in any of my posts to you in any way a reflection of Buffalo Tech.