I would like to know this also, you could use Add2PSU, that would make 2 psu connected, I have not seen more psu connected by using more Add2psu device, maybe someone has tried it?
You can try this technique:
The simple way to do it is using jumper technique. Use conductor material to connect PIN 17 (if u are using 24 PIN) with ground (PIN 3/16/18 etc) or Green with Black.
or using a tools like this:
http://www.legitreviews.com/add2psu-daisy-chain-power-supply-adapter-review_1698