Here is what you want to do:
- Buy this upgrade bundle form OWC, this bundle has the blade SSD which is the proper SSD you need
- Follow the guide I linked to below, you can keep your old HDD and have a blade SSD running simultaneously
iMac Intel 27" EMC 2546 Blade SSD Replacement
The upgrade bundle from iFixit won't work because it comes with the wrong drive which could only go in the place of your current HDD. I understand you want to keep your current HDD, so the kit from OWC will be your best option because it comes with the blade SSD instead of the SATA SSD.
It appears that iFixit doesn't sell the right blade SSD for your iMacunfortunately. Again, OWC is your best bet because of the drive compatibility.
Added by @mayer: (thank you! I would have upvoted that comment if I could)
Here’s how to set up a fusion drive on your Mac