If a HP workstation is properly W7Pro64 activated, whether that activation is because it is from an official HP W7 OEM license, or if you bought an official W7Pro64 from Microsoft (for example, as a "system builder" DVD via Amazon, etc.) then you can upgrade that to Windows 10 Pro 64-bit for free via the online Microsoft system that supposedly will end end of July this year.
What I do is use Acronis to make an image of my W7 install, clone that onto a "stunt" SSD usually of 160GB size, keep the original drive safely set aside, do the W10 upgrade onto that clone SSD (I choose the save-nothing option in the process). I use the Microsoft system of online creation of a thumb drive with the W10 installer, and don't even boot from that thumb drive at this stage of the project (just navigate to the .exe installer file and run that from W7 or W8).
This results in your workstation thereafter being perpetually W10 activated. Then, I'd do a long version of reformatting the target SSD that I want to load W10 onto for that workstation, and start the W10 load from scratch. The Microsoft system of W10 loading probes your hardware and loads what they have found to work for the motherboard and subsystem components. Of interest, if you have a bunch of those workstations you can do this process of a clean load once, and clone that onto all the others (assuming that you have W10 activated each of the others already).
So, just because you don't have a HP W10 installer that does not mean you cannot load W10 and even clone your gold standard build to other identical or similar workstations easily. I've been doing this with the HP xw workstations for over a year now. You'll only be able to know if this Microsoft based approach works for your hardware by trying.
It sure works great for our xw6400, xw6600, xw4600, xw8400, xw8600 and Z620 workstations.
