The best, and really the only way to do what you're trying to do is to build a custom version of FreeNAS with the driver(s) of your choice compiled in.
It's easy to build FreeNAS. Just instantiate a stock FreeBSD 9.2/9.3 build host, fork the appropriate repo on github, add your driver to a branch, modify the checkout script to use your branch instead of the stock one(s), "make release" and bob's your uncle, you'll have an ISO and upgrade image to use which contains the updated driver or whatever other custom bits you want to include.
If that previous paragraph is over your head, then this is like riding the Space Mountain roller coaster at Disneyland - there's a sign which says you must be a certain height to ride it, otherwise you get to turn around and ride the tea cups or the merry-go-round instead. Same principle applies here. I'm not actually trying to be condescending or discouraging, either - this is literally the level of skill required to accomplish the task, nothing more and nothing less. FWIW, there are also hundreds of people who have started with even less information and, using google to provide the details (repo URLs and such), popped out a custom FreeNAS image several hours later, so it's not rocket science either.
