The 2011 iMacs come with Radeon GPUs. The motherboard contains a custom daughter card that contains the GPU. Theoretically, a different graphics daughter card could be put in. But Apple never made one. So someone would need detailed design information about custom add-on cards from Apple in order to create a product with an NVidia GPU.
The iMac contains a custom-designed logic board. It isn't like a PC where you can just swap a card into a PCIe slot. There are no standard PCIe slots in an iMac. It's all mobile chipset custom designs. Only Apple could make an "upgrade" and they are interested in you buying a new iMac, not upgrading an existing one.
The only "upgradable" Mac that could be user-modified like you want would be the Mac Pro.
Look on iFixIt for a tear down of the 2011 iMac and you'll see how almost impossible it would be to replace the GPU with a different one.