Hi
You have a really cool system! 
So it is not possible to remove the back panel, no thumb screws?
I looked at your system specs at this site but could not determine if removing the back panel was possible.
Note, you need Flash to view all contents at this HP Support site.
HP has done some nice cable management in your system.
It is hard to see how many SATA ports are available on the MB. Do you have an open SATA port for the additional HDD?
You may be able to remove the back panel and then route the SATA cable to the open HDD bay. Or can you remove the HDD enclosure to route a SATA and power cable to the open HDD bay?
Have you run the final fantasy 15 benchmark?
Tom
My PC's:
Asus Maximus X Hero
NZXT S340 Designed by Razer
i7 8700K
1TB 960 EVO (NVME)
Asus Strix 1080
Corsair RM650x
Kraken X62
Asus Z170 Pro Gaming Aura
Corsair 450D
i7 6700K
1TB 850 EVO
Asus Strix 1060
Corsair RM550x
Kraken X52