Yes you CAN install an AirPort Extreme card in the first gen Mini, but it's a pain. I don't have my notes with me right now, but it's going to involve basically disassembling the Mini to the point where you can get to the subframe that houses the card-and I don't remember if it's the same Express card that goes in iBooks or not. Hit Google for something like "install airport mac mini" and see what that turns up for you.
As Macola says, most USB devices don't have Mac drivers, so they're a problem. You could search back in this forum a couple of months for the USB devices that people HAVE found to work - with some special drivers and a few dodges. The best choice is to use a real AirPort card, but as I said, it's not going to be easy.