The problem are the drivers for the tuner cards. I had the same idea a few month ago and FreeBSD itself supports my card and have a driver for it.
BUT in FreeNAS they removed all drivers wich are not necessary for a normal NAS so you have raid card and lan card drivers but nothing for a TV card. -> FreeNAS 8.3.1
I don´t know whats about the drivers in FreeNAS 9...