Precision 3561 - 2.5in device

Hi! I've got a Dell Precision 3561 with the small battery (64Wh). Therefore I still have enough room in the device to add a 2.5" SSD or HDD. These are even sold via the official parts store (https://www.dell.com/en-us/shop/pfydresults/272652). What I cannot find however is the adaptor cable and the HDD tray/fixture I will need. 

I am well aware that I could just get an M.2 drive, but I still have a 4TB SSD which I would like to continue using. It's just for backups and additional storage, I don't need lots of performance, so SATA is fine.

Pro Support told me it's impossible to use 2.5in devices and to get a M.2 SSD. I find that hard to believe however, since it's mentioned in the docs and the port is available in BIOS.

What if I ordered the 2 TB HDD from the shop (as an officially available addon), this would also need to be connected somehow. Please let me know which cable and tray would be used to install this HDD.