First the Apple blade SSD is removable, its just very difficult to get to. Review this IFIXIT guide: iMac Intel 27" Retina 5K Display SSD Replacement.
As to swapping out your current HD for a second SSD, yes that is something you can do. You will have a few issues here. First thing is replacing the HD's thermal sensor, while the replacement SSD doesn't get as hot as a HD you still need to add-in a replacement thermal sensor like this one: OWC In-line Digital Thermal Sensor for Hard Drive Upgrade for 27" iMacs 2012 and Later. In addition you'll need an adapter frame to mount the 2.5" SSD into the 3.5" HD frame. Here's what I recommend: OWC 2.5" to 3.5" Drive Adapter Bracket. Here is the IFIXIT guide you'll need to follow: iMac Intel 27" Retina 5K Display Hard Drive Replacement.
I must warn you, you can damage the display assembly if you don't use the correct tools and techniques as written in the IFIXIT guide. This could be a very expensive upgrade if you need to then need to replace the display! Last thing here is make sure you have broken the Fusion Drive set before you replace the HD otherwise you can encounter problems.
You may want to think about using an external Thunderbolt2 RAID drive setup. Depending on what you are looking for you might find this a cheaper and safer upgrade. Here's one unit: OWC ThunderBay 4 mini. We have a few of these and find them quite effective.