27" iMac can I upgrade to a fusion drive later

Officially, no; the drive you get when Apple builds the computer is the drive you're stuck with.

The Fusion Drive consists of two physical devices: A traditional spinny metal platter hard drive, and a solid-state NAND cache, similar to an SSD. CoreStorage allows you to format the two physical devices as a single logical volume using Disk Utility; the resulting volume is the Fusion Drive, which stores the most frequently accessed data on the SSD (such as the operating system and applications), and stores the less-used data on the platter drive. In the Mac mini, the two separate drives used to mount in stacked SATA bays, similar to the stacked hard drive cages in towers.

Apple has a connector for the PCIe solid-state drive module on the logic board, which in Fusion builds is used for the NAND cache that's mooshed together with the traditional SATA hard drive to create the Fusion Drive logical volume. But there's a good chance that the SSDs Apple uses have special firmware; that's what they did with the 2013/2014 Macbook Pro/Air laptops, which use similar PCIe modules. If you stick a generic third-party module into the socket, you may not be able to communicate with it.

Unofficially, it's hard to say. I haven't read any reports of someone trying to install such a PCIe module and failing, so who knows? Maybe Apple uses a different implementation on the iMacs. If you're inclined to try an experiment, iFixit has a guide for removing the SSD in a 2013 27" iMac; I'm guessing that the logic board of the 2014 non-Retina generation is similar. From the looks of the socket, the module uses an mSATA connector. But you'll only know whether it works once you've reassembled it.

There is another alternative. About a year ago, Western Digital released a 2.5" hybrid drive with 120GB NAND cache + 1TB platter drive in a single 2.5" laptop drive mechanism, under the name Western Digital Black2. Until recently, this drive was not supported under OSX. A tip from DealNews about NewEgg selling this drive for $120 drove me to Western Digital's website, where I discovered that not only is the Black2 now supported under OSX 10.9/Mavericks, but Mac-based firmware patches and configuration utilities have been released. Configuration options include Dual Drive (treating the 120GB SSD and the 1TB HD as separate devices) and Fusion Drive.

Yay!

The WDC documentation specifies 10.9/Mavericks support, but that document was written in August 2014, two months before 10.10/Yosemite was released. I suspect that, if the drive actually works under Mavericks, it will work under Yosemite the same way.

So, instead of messing around with possibly incompatible SSD modules, you might try replacing your existing hard drive with a Black2, and get most of the Fusion functionality that way. You'll also need to get a mounting bracket to install the 2.5" Black2 SATA drive into the existing 3.5" drive bay. It will be exciting to see if this works; I'm planning to try it in a laptop just to see.

Excelsior!

2015.05.10 EDIT: I have just discovered that WDC has updated the install notes and OS X utilities for the Black2 SSHD; formatting as either separate SSD/HD volumes or as a conjoined Fusion Drive is now supported for OS X 10.10/Yosemite.