Your system comes with two ports! A SATA 3.5” HDD port and in your case an already filled PCIe/NVMe blade SSD port.
So yes! you can add in a second drive. A Fusion Drive it just that using both ports. But unlike the Fusion Drive which has a tiny SSD you have a much larger one. Even still I would leave them discreet and not setup a fusion drive set as that would be wasteful of the all that space your blade SSD offers.
So you’ll need a SATA cable and adapter frame in addition to the 2.5” SATA drive you put in as well as new adhesive tapes. To make it easy just get one of these kits: iMac Intel 27" (Late 2012-2019) SSD Upgrade Kit and you’ll need this cable as well iMac 27" Retina 5K Hard Drive Combo Cable (L15/17/19) Apple P/N 923-00664
We don’t have an explicit guide for your system, this is the same! Reemplazo de Disco Duro del iMac Intel 27" pantalla retina 5K.
Be very careful taking the display off as its very easy to damage! Use pizza cutter tool within the kit to carefully separate the display from the case. Make sure it doesn’t slip off as you can damage the cables which connect it on the back.
Once you get the drive installed & formatted, Transfer your data from the Blade SSD over freeing up as much space as you can on the bootable blade SSD. As your OS will leverage this free space and as its the faster I/O this is the better drive for it to use for virtual RAM, caching and scratch space if your app uses it.