No, the USB port is only for USB storage and printing devices. It doesn't have the drivers or code necessary to handle other USB devices.
If you want a MJ thin client, you can typically find plenty of them on eBay. Just search for “magicjack thin client” (no quotes). And yes, they’re not necessarily cheap, although you can occasionally get lucky and find something in the $50-60 range.
That’s why MJ isn’t always the best option if you don’t intend to leave your computer running 24/7. There may be better VOIP options that already include a standalone device (e.g., OOMA).