They add phases, bridge phases from other motherboard into one, etc.
Have you never seen that?
Yeah, that's how they double current, not voltage.
The point is that you would need to at least replace all of the caps with ones that support twice the voltage (which could easily translate to caps that are twice in size.) Now that you've swapped out the caps you would need to hack the BIOS because the caps
will burst if you even attempt to run VRMs at this voltage. Also if the capacitance of any of the different phases changes, you need more than a custom bios, you need to update it to handle the hardware changes you've made.
Also the addon board you speak of I've only seen for Kepler-based GPUs since voltage control is locked. I've yet to see this for the motherboard and CPU power phases, but once again there is a different reason for that.
This was a suicide run, obviously, and they'll do crazy shit just to get a CPUz screenshot before the system dies.
It would have died the second it booted with a voltage like that. LN2 doesn't let you overcome physical limitations like the breakdown voltage of the caps. You'd pop the caps (haha!) before the VRMs could even charge.