Converting passive USB Hub to powered USB Hub, by adding regulated wall-wart as power supply?
The (externally) powered USB hub I have just connects the power supply to the appropriate pins of the USB connectors. Power from the PC is connected trough a diode, so the hub can be powered by the PC or the power supply.

USB specification says that devices should negotiate for the power with the hub or PC, but in practice, just connecting the 5V to the required pins works just as well, especially if the power supply is powerful enough for all devices. Some USB devices (battery chargers, lamps, fans etc) do not negotiate for the power, just connect the power pins to whatever is needed.