the design of nuclear reactors, and as Syed.Ali.Haider mentioned you also need a heavy water. Which was hard for us to get hands on as after India had tested it's device the Canadian's didn't sell us heavy water. If i am not wrong it was acquired through China, or we produced it ourselves. Not too sure. Say if you do make a reactor that works, once you have enriched the Uranium you don't really have a nuclear bomb ready. Requires Fission, which in itself requires more of mechanical experts to create a device. Getting all the mechanical and electronical needs in place, you also need something to deliver it. Probably need a missile or something, i guess. So i don't really get your comments.
Secondly, before giving all the credit to one man you should know that A.Q khan himself wasn't alone in efforts to enrich uranium. Not to forget the efforts of DR. Munir Ahmed Khan.
Apparently it is not the only step. That being said i will not undermine the role of A.Q khan, he did his part like the others.