What, specifically, did you need from FreeBSD? OS X does have some BSD code in it.
It sounds like you want to do the compile on your Mac, and then have a binary come out that will run on FreeBSD. Right?
That IS possible. Last time I did it (Linux compiling for AIX) it took building GCC from source and then the various libraries. It was not a small effort. Today, I would agree with the suggestion earlier to install FreeBSD on a vm and compile there. Much more straightforward and fewer headaches.