This was inspired by some (read: a lot) tutorials about "How to charge * on USB power", so I reversed the logic and I'm publishing "How to connect *'s charger to USB ports".
As an "added bonus", you can either use the 2 USB ports only with power capabilities...
... or you can connect them to your PC and use them for both power and data transfer.
Step 1: Materials
USB card with cable (transforms internal USB connectors in the standard A-type (rectangular) connector)
Connector that fits the USB card
Power supply: 4,75-5,25V, at least 500mA (recommended: 500mA*number of outputs)
Soldering tools/Glue/Scotch/Whatever holds wires together
Step 2: Cable Modding
Identify the power lines, usually the outside pins (In my case there are 2 rows of 5 pins each, but usually you will only find 8 pins or 9 pins, where one is not connected).
Cut off one side of the 2nd cable, and clearly mark the corresponding pins.
Step 3: Connecting the Power Supply
Since I have a charger that outputs 450mA* at 5,2V it would be a good choice for this project.
Simply connect the power supply to the modded cable... just get the polarity right and there shouldn't be any problems.
*not enough for safe operation!!
Step 4: Test
Check for 5V on the actual USB port, and test for any shorts.
Step 5: Use in "power Supply Only" Mode
Plug in the charger, connect the output, everything should be up and running.
Step 6: Use As an Actual USB Port
You can plug the original cable in your motherboard and to the USB card to also get data capabilities.