Did you try using the "Search the community" search box just above? It defaults to searching only this forum, which is handy.
For example, I just searched for Z620 Pinout, and this may help:
The pinouts have stayed similar over the different generations of workstations so you may find added info from prior posts too. Dan is a very helpful HP engineer who has been able to provide lots of pinout solutions. If you need me to get into one of our Z620s to take a look let me know.
Also, be sure to get the "2 x 2" version of the HP USB 3.0 card, which is the later one based on the TI chipset. It has 2 ports on the rear, and the header on the card for two up front. It is best to put that PCIe card into one of your PCIe generation 2 slots because that will have the appropriate bandwidth. You want a Gen 2 slot to match USB3 speeds, and in a xw6600 only the two video PCIe slots are Gen 2. I can't remember which of the PCIe slots are Gen 2 for the Z600, but that info will be in the Z600 Maintenance and Service Guide which you can find via Google. Sounds like a great project.
Many 4-pin to SATA power adapters don't fit well on that card, but the Molex brand ones from Mouser.com do fit perfectly. If you get the full kit from HP it will have a power adapter that will fit fine, but if you buy the card bare you'll need to get one. Best to tap into a 4-pin spare power connector up by the optical disk area if you have one, so you may also need a 4-pin male to 4-pin female extension. For me in the xw6600 a 12" extension plus the Molex brand adapter gave enough room to get the power feed down easily.
Finally, power the card before you attempt to load drivers...... parts of the card won't be seen by the driver installer until you get it power from the 5v feed from above. The PCIe slots don't provide that.