I'm not sure you can...
I know that you can't go from HDD to FusionDrive, because your logic board is not equipped to accept SDD on this model when you buy it with HDD.
Sorry for the incomplete answer.. ;-(
Yes you can....
Kodawarisan had open the new iMac.
there is a slot to insert a strip SSD Apple even on the entry-level model that does not default, it will still find a way to get them or someone so this adapter connector to the SATA classic...
See opened iMac pictures from Kodawarisan
YES YOU CAN,
but we don't know if the fan will spin to out of control because of the fact that the sdd won't be from apple. so we might have to cut the fan wire from the sdd
and also the fact that apple does not enable trim for a third party sdd, so if you want the sdd to go to full potential for your new iMac
you have to download a third party software such as TRIM Enabler to activate it.
this is the sdd that I am putting in mine
according to this article the 27 inch models are here
but i think that the 21 inch is a better deal even though you have to remove the fan to change the ram
were as the 27 inch has ram slots in the back under the stand.
and the only tool you need to open it with is guitar picks so don't buy the heat gun or suction cups it is a wast of money! If you don't believe me look at this
The problem is not to open the iMac, and yes, I think there are no problems with an SSD instead of the built-in HDD, according to some experience on the web it will take about 10 minutes to open the iMac, take away the HDD, replace it with an SSD, and make it work.
The real "problem", is to replace the glue stick to close the iMac properly....
I'm going to buy this iMac, and I'm thinking about the "adventure" to replace the HDD with the SSD (like I did on my old Early 07 MBP... (and no, there are no fan spinning crazily or other problem caused by trim or not, and however there's an app called trim enabler...)... But I'm really "scared" about this thing of the glue sticks to replace the original glue...