Cut the original PSU (Power Supply unit) so you have a usable length of wire coming from the Z2 power connector, and attach the USB male end. Make sure you dont get the polarity wrong! If you look closely there will be one wire that has a white stripe on it, for my PSU this was the negative lead. I highly suggest using a voltmeter to make sure.
If you need the pinout to the USB port check this URL for more information.
In the end you can charge the Z2 using a USB port, the original PSU, any generic USB Wall Charger, or even use an emergency USB Battery pack, like those found in stores to boost the battery life of your cell phone or MP3 player.
The final product.
This is not very hard if you have the parts laying about and some basic soldering skill. I was able to complete this in about 8 minutes. It took me longer to heat my iron than it did to actually build.