This is one of those "if you have to ask, you shouldn't do it" situations. Theoretically, it's possible. But since you can't buy something to do it for you, you'd have to build it yourself. Meaning you're an electrical engineer and wouldn't be asking the question in the first place.
I'd also strongly discourage buying these high-Wattage AC adapters off eBay. Anything over about 180 Watts is probably an unlicensed product straight from a Chinese factory. In the U.S. and EU, electrical devices like this need to comply with government regulations for safety and to limit RF emissions which can interfere with radio, TV, WiFi, and emergency (police/fire) radios. The ones from China could cause a host of RF problems, or even represent a fire hazard (the ones I've see are very cheaply made).
PicoPSUs are able to work at lower wattages because the waste heat from the AC-to-DC power conversion is small enough to be dissipated by simple radiation, or with a small fan mounted to the power brick. But once you start getting to higher wattages, even a 80%-90% efficient conversion results in 10%-20% waste heat. For 400-500 Watts like you're proposing, this is about 40-100 Watts. That's as much heat as a modern CPU under full load, and you need a similar power fan to dissipate that much heat. Which is why computer PSUs are as big as they are.
You may be tempted to get three PicoPSUs (and three power bricks); one for powering the GPU, one for powering the CPU, and one for powering everything else. But powering with three independent power sources like this may result in small voltage differentials causing unwanted current. e.g. Your GPU PicoPSU is delivering 11.9V while your motherboard PicoPSU is delivering 12.2V (the voltage delivered drops slightly with load). So now you've got a 0.3V difference between the GPU and motherboard, causing current to flow between the two. This could cause system instability or even fry your hardware, so I wouldn't recommend it. You'd need to add some sort of voltage regulation to the system, which gets back to "an electrical engineer wouldn't be asking this question."
I suspect most EEs would be reluctant to offer DIY instructions over the internet. 400-500 Watts and circuitry which modifies voltage can easily kill you if you screw something up. The reason 3.3, 5, and 12V are relatively safe is because the voltage isn't high enough to create enough current to kill you. But once you start messing with modifying the voltage, all bets are off. If you really want to pursue this and have an EE friend, see if they're willing to help in person. Otherwise, do as bliq suggests and go with a regular (but smaller) PSU.