I would say almost certainly no. Your chipset has to support it, and it's a different brand and generation. I'm not sure where you can even go to buy standalone mobile graphics cards on the Internet. I'm sure you can, but I couldn't find a place.
You could buy a new iMac with one, but that would sort of defeat the purpose. If you really want it, I would suggest selling your current machine ASAP before word of the new ones gets around too much.
People have noticed that it looks like the GPU is on the motherboard in the 2012 iMacs as well, thus not swappable by itself.
