Hi
If your desktop has some free PCIe slots then you can easily add an additional USB 3.0 controller in the form of PCIe card. There are options with 1, 2 or even 4 controllers on a single card available in the market. Our website lists some recommended models of PCIe USB cards that work well with PS Eye cameras (look in "USB controllers" row in "For Web Cameras" section).
http://ipisoft.com/store/hardware/How many cameras were you able to connect so far? Usually modern Intel USB controllers (which combine USB 2.0 and 3.0) can handle 4 cameras without a problem. So you probably need only 1 additional controller. However, you may decide to take 2-controllers card to have a room for upgrading to more cameras later.
FYI you can use more cameras in lower FPS modes, e.g 3 cameras per controller at 40-50 fps.
