I would like add a connector for a hard drive, is that possible?

I presume there is a 2nd hard disk bay next to where the SATA connector would go?

If your soldering skills are up to it, why not give it a go? Check out the options that come with that model. It may be that it's optionally shipped with a relatively modest SSD for the OS plus a much larger HD for data.

However, it could be that a firmware upgrade is required. The cost of a SATA connector is so low that I'm surprised it isn't fitted as standard even if not used, in order to eliminate the need to hold stock of 2 different versions. So it could be that it was a twinkle in the disigner's eye that was never fully implemented. Often you'll see spare unpopulated pads on a motherboard for some option or other they thought they might have implemented but didn't.