MtothaJ is correct.... you're worried about a non-issue. ATX standard is 75W per PCIe 6-pin supplemental power cable. HP workstations give more by quite a bit..... 18Amps x 12VDC = (you do the math). nVidia is referring to a standard ATX power supply, not the HP workstation power supplies built to higher-than-ATX standards.
This is why HP sells their adapters we have posted about here that let you do things with their PCIe power cables you cannot do with an ATX-standard power supply's cable.
Just get a good adapter with all or almost all of the pin positions populated. You don't need to only buy a HP one, just a good brand name one. You'd be fine to use that Evga aggregator adapter M linked to, but you don't even need to use both of your Z420's 6-pin supplemental power cables for that.
