TLDR; yes, it is possible to add an SSD to a 2011 27" iMac without going through too much hoopla. Between everyone having issues finding parts, non-workable guides (due to parts), and general ...

But if you’re going to have both the SSD and original hard drive in your Mac, you could pick a smaller SSD for OS X and your apps, keeping your music, photos, and videos on the old hard drive.

So what if you've done everything you can, but your iMac is still sluggish? Grab a speedy external SSD and do an OS X brain transplant!

A disclaimer: if you bought a 21-inch iMac with a standard hard drive, you’re out of luck—these iMacs didn’t have an PCIe SSD slot on the board.