5 V from 12 V at 650 W?
Sure you can. There are several options:


Get a resistor and a giant heat sink, if your load is constant.
Get a Switiching regulator from 12V to 5V, at 650W, that's still going to need a heatsink.
Realise option 1 and 2 are more expensive and inferior to buying an off-to-shelf 230V to 5V power supply
Try to mess with your current power supplys regulator to get 5V out of it, and in the process get a nice feel what 230V through your body feels like (hint: it's not great)