The NAS box usually has just a wired network outlet. In my setup that goes directly into the wireless router. If you can tolerate the noise of hard drives spinning in the room the router is located, just put the NAS box there as well.
If you can't, it _may_ be possible that the NAS box supports (some) USB WiFi sticks (after all, it probably runs some variant of Linux), you'll have to check explicitly for that. If it doesn't you'll probably need an
access point for it.