Can a USB 2.0 cable support USB 3.0 charging?

I think the answer you are looking for is all about the cheap cables some companies supply, and if they can cope with carrying the full 900mA.

No, not all cables are capable of carrying the full 900mA. It will depend on what size the wires are in both diameter (gauge) and length. The length is seldom considered when using cables. If you try to draw 900mA down a short cable then you may be able to, but if you try to draw the same 900mA down a 3 meter cable then most likely you will fail and it could even cause a fire by either the connector or the cable getting hot.

Some of the makers of USB cables try saving money by using wire so small that it cannot cope with the extra 400mA difference between the 500mA of a USB2 and the 900mA of USB3.

If you do go ahead, then make sure you test to see if the cable or plug gets hot after using it for a while. 10 to 15 minutes at least, and do not just plug it in and then come back after the time is up - make sure you keep an eye on it for the full duration. I personally keep one of those laser thermometers next to me to check the temperature of the wire and plugs before starting the test, and the temperature as time passes.