One way to do it is to remove the rectifier diodes and take the AC from the alternator windings into three transformers which step up the AC voltage to whatever you want. The output voltage regulation will not be good, and the frequency will be very high, and all over the place as engine RPM varies.
You can however rectify the output to create high voltage dc, and use this to control the alternator with some additional electronics. I did this years ago to generate +1200v 400mA dc, to power a mobile amateur radio transmitter before high power RF transistors were cheap and available as they now are. So it can be done.