USB port modules certainly can be upgraded, I did it on a load of Antec cases when USB2 came out just after I did a large number of builds. I contacted Antec and they sent me replacement case modules (no charge). I disassembled the case front, took out the old modules and replaced them with the new ones.
If you look at many of the "old favorite" cases on the market, you will see them with a "V" suffix....two in point are listed below:
Antec 902 V3
http://www.newegg.com/Product/Product.aspx?Item=N82E16811129097
Antec 1200 V3
http://www.newegg.com/Product/Product.aspx?Item=N82E16811129100
The "V3" indicates the cases have front USB3.0 whereas the older ones w/o the "V3" designation have USB 2.0
I'd contact your case manufacturer and offer to buy the parts for the upgrade. Maybe they will send for free as Antec did in my case years back.
Two qualifiers....
1. Neither the USB 3 port you have or the one you add will be worth beans if ya MoBo doesn't have USB3 headers to feed them with
2. The USB module may be a different size than the two making this effort a bit more complicated than you'd like to undertake.....especially since it appears ya have no USB thingies to plug into it.