Your first misconception is that xnu (the OS X kernel) derives mainly from BSD. In actuality, it's based much more upon Mach than BSD. The BSD subsystem is largely implemented on top of Mach subsystem in the kernel (e.g. a BSD process is implemented on top of a Mach task), and xnu supports both Mach and BSD system calls. There is very little you could port from BSD into xnu without significant work, although obviously there has been a lot of cross-pollination between xnu and BSD (launchd, GCD, mandatory access control, etc.).