Can I use a USB 2 charger with a USB 3 cable

Yes it does, in terms of polarity and voltage, but not current. Some chargers short pin 5 (iirc) in order to tell the device that "hey we're breaking the USB standard here and you can draw more current."

Motorola phones will happily send 750 mA down the line, Blackberries 800 mA (that may be reversed). Usually extra current is just extra capacity and you're fine--it won't hurt a device. Under-current, however, will most likely fail to charge. Poorly designed devices may malfunction.

The USB 2.0 standard is for a device to draw 100 mA--and then ask for more current; most devices don't follow that part of the standard. (I have a device that does...and it has a mode that says "draw 500mA no matter what")

USB 3.0 will increase the currents allowed to 900 mA, and allow 150mA inital draw.

On page 33 of the motherboard's manual you can find the pinouts for both USB2 and USB3. They are not compatible. And even if they were compatible the physical connectors are not - the spacing between pins is different.

There are 2 "sockets" in each connector, so if you really, really want to connect USB2 cable into a USB3 socket you have to match pins manually:

• USB2 Power (5V) to USB3 VBUS
• USB2 USB DX- (or DY- for second socket) to USB3 D1- (or D2- for second socket)
• USB2 USB DX+ (or DY+ for second socket) to USB3 D1+ (or D2+ for second socket)
• USB2 GND to USB3 GND

This is a dangerous procedure so if you don't know what you're doing just buy a separate controller and use that.