Upgrade/add (if I can) an SSD instead of my HDD

Review the guides below very carefully, it is very easy to damage the display assembly it you don't use the correct tools and use care!

Here is the IFIXIT guide to replace your HD (in your case swapping it out for a SSD): iMac Intel 27" EMC 2639 Hard Drive Replacement In addition you'll need the following:

• OWC In-line Digital Thermal Sensor for Hard Drive Upgrade for 27" iMacs 2012 and Later as your new SSD won't have the needed sensor built in anymore.
• OWC 2.5" to 3.5" Drive Adapter Bracket to make up the difference between the 3.5" HD to the new 2.5" SSD you are putting in.
• And lastly the SSD drive its self.

Here is the IFIXIT guide: iMac Intel 27" EMC 2639 SSD Replacement As you can see adding the blade SSD is a lot of work. You also will need to find a compatible SSD unit, so far Apple is the only suppler either as a spare or from a system as a parts teardown. There are a few 3rd parties hinting on offering their own solutions in the spring. So for now it maybe wise to hold tight.

As far as setting up a Fusion Drive set you'll need to have both your current HD and add in the blade SSD. The idea of a fusion drive is having two physical drives act as one in this case both an HD & SSD. That way the OS can manage the storage space putting more needed files on the faster SSD like the OS and some of your apps.

Another approach is to use a SSHD which is a single drive that has a SSD acting as a deep cache for file access. Here too the SSD holds the more often needed files and the traditional HD space holds the majority of the files. Here's one example: Seagate 3.5" Desktop SSHD. You will still need the external thermal sensor: OWC In-line Digital Thermal Sensor for Hard Drive Upgrade for 27" iMacs 2012 and Later. Frankly, if you need to do this now this is the best option in my opinion.