It has two mini PCIe slots, one for mSATA, one for wifi. It has the usual antenna holes as well. You need an mPCIe card (Intel 7260HMW is popular) and two U.FL to RP-SMA connectors and be done. But yes, you could use USB as well. They sell kits as well but they use inferior cards IMO.

Thanks! I have an ASRock Beebox which has mSATA WiFi card installed by default, but I've never trusted the signal due to its lack of external antennas. I was wondering how well it performs in your setup :)