Swapping out HD adding blade SSD

Simple answer, No.

Presently these super HD's are not rated for desktop use. They are designed for deep storage systems like a NAS or SAN system (RAID'ed). You face three issues here the physical size of the drive (height), the amount power the drive needs and lastly, the amount of heat it creates. So far these drives require too much power & create too much heat for the iMac.

Frankly, I don't think its wise putting all ones eggs into one basket here. I strongly recommend getting an external Thunderbolt drive array (RAID'ed) then you can put in your 8 or 10 TB drives (2) into it as a RAID pair. Gaining the massive storage and the high speed interconnect. Then if your system has a problem you can swap it out without loosing a beat!

OK, now the blade SSD:

This system uses a still newer version of PCIe interface! The older models uses a 2 lane interface, now Apple has bumped it up to a 4 lane which is Good! But no one offers a Apple spec'ed blade SSD for this system at this time. The other factor here is its a lot of work to open the system to even gain access to the SSD connector. Review this IFIXIT guide: iMac Intel 27" Retina 5K Display SSD Replacement. Note the first 23 steps is just taking off the display assembly! The next 30 steps getting the logic board out just to get to the SSD.

Bottomline here: You'll need to wait until a 3rd party offers an upgrade option that may take a while. If you just got the system (a week or so) see if you can return it and get a system with a SSD already installed and what ever HD drive Apple offers with that system.