According to the specs on your mb you have 6 usb ports on the back, and 6 internal.
Your are probably using 2 of those for your front panel connection so you should have 4 left. they come in groups of 2 so you should have 3 headers on your mb.
Your board should have come with a pci slot shield with usb ports and a cable to connect to the mb header.
I would go that route first as it is no additional cash outlay.
yep I see it upper right.
In perusing your manual it appears to only have 4 onboard. You are probably using 2 for the front with 2 left that you can use the adapter for. after those are used, pci card or hub.