All current iMacs have these soldered on.
In older iMacs you can switch MXM cards, but if you read the existing thread, people have had pretty poor luck.
At least with Nvidia cards, Apple uses ones that are frequently a byte different in device id. So the BIOS would not work without some pretty intense surgery.
The way to find out would be to compare the device id of the BIOS.
But even then it is an unknown if there are any other differences.
There is a thread here where I offer to help someone trying this. I am much more fluent in Nvidia EFI/BIOS modding. Nobody has sent me one yet.