Pcie cables = 12v + ground.
EPS cables = ground + 12v.
20+4 cable = 3.3v, 5v+, 5vvsb, -12v, 12v, power on, grounds etc.
So no, you cannot just use adapters or other cables to supply the 20+4 mains for the motherboard, doesn't work that way. The psu could have 4pin outlets or 12pin outlets, that doesn't matter, that's just a physical connection of the cable to psu. What matters is the motherboard end of the cables as those need to match up power to power, ground to ground etc. Putting psu ground to motherboard 12v or vice-versa when more than one power wire goes to the same plane = dead short = non functional psu if it has good protections, or blown up motherboard/psu/gpus/cpu/ram if it does not.