Yes you can install a standard sata 3.5 inch hard drive into the top caddy and plug it in.
The Omen should have come with a package of screws for it that you will need.
Boot up the PC and it should recognize the drive the same as if you were installing it in an internal bay with a sata cable.
Note: its not like plugging an SD card into the SD card slot. The PC should be off when installing or removing the drive from the bay. If the drive is plugged in and you remove it while Windows is booted, you could risk corrupting the data.