USB: clean, regulated 5volts.
Car: Unregulated 10.5-15 volts.
Any usb device taking in 12 volts will fry.
Usb has 4 cables inside of it. Red, black, green, and white. Green and white are data. Black and red are power.
USB ports are limited to 500mA.
A usb hub uses 100mA to power itself. Then gives the remaining power available to the other ports.
However you shouldn't push more than 500mA on a usb cable. This is why powered hubs exist.
The data cables cannot be joined. A hub actually has a lot of electronic stuff going on.
What you need is a powered hub powered by either a 5 volt regulator or by the computer power supply's 12 volt rail.
From there you can plug in extensions to make it look OEM