Please see the attached info from HP regarding installing GPU cards;
Also, if you check with the graphics card manufacturer you may be able to find some additional info;
https://www.nvidia.com/docs/IO/43395/tesla-m2090-board-specifications.pdf
specifically;
(The above link is for my Tesla M2090 GPU card).
In summary, yes you can use a 6-pin to 8-pin adapter but make sure you do not exceed 225W power consumption.
The Z620 has 2 off 6-pin PCIe connectors and these can be used to power either a single GPU (i.e. paired together), or 2 separate GPU's. Each PCIe connector can deliver 150W of power (with the PCIe slot providing the additional 75W of power).
I bought my cable from Maplin;
Currently in the process of installing a Tesla M2090 GPU card (225W) to compliment the existing quadro K4200 card (108W). The Z620 should be able to handle 2off 225W GPU cards.