External GPU for iMac

Hello

I've read a lot on the internet about external GPU for the Mac. I've seen people have external cards for their MacBooks, and I'm wondering if I could do the same with my iMac. Most threads and blogg posts have said that this is impossible, but I want to know myself if it really is.

I got an iMac 21,5", late 2012, OS X 10.10.2. The current card is a NVIDIA GeForce GT 650M 512 MB. What I'm thinking is that the card could be connected by ThunderBolt. If it doesn't work for the iMac screen, I got a second monitor that I use. Maybe it can be applied to that monitor? If the problem is that the OS won't recognize the GPU, I could use my BootCamped Windows in order to have the GPU. Anything would help.