its actually physically impossible to charge through usb because
1. not enough power goes through usb to actually charge it properly/would take too long
2. usb isnt connected directly to the power on the battery.
but with some technical knowhow, you could resolder yourself a usb-power cord thingy but it would fry in a matter of miliseconds as the power cable inside the main usb cable (theres 4 tiny 1mm cables inside the main big covering) is only 1mm thick and it wont take much to fry it
Inside a USB cable: There are two wires for power -- +5 volts (red) and ground (brown) -- and a twisted pair (yellow and blue) of wires to carry the data. The cable is also shielded.