Hey, langmead!
What a beauty you got there!
After checking out the specs of it, I saw that on the official website it is stated that there is no option for additional HDD.
However, after some more digging I found that in this port you found, you can put a secondary hard drive but utilizing it requires a modified SATA cable as well as a caddy.
There are specific HDD caddies for these all-in-one computers, so it is possible to maybe replace the ODD (it is SATA) with the second HDD and that's how you supply it with power.
You can google and see for yourself!
Hope this helps.