As bent christian said, replacing the HDD with a large SSD would probably be an easier and way cheaper solution than replacing the proprietary SSD in your iMac. If you are willing to slice up your iMac and reseal it again it is definitely possible. I have done it on a late 2012 iMac where I replaced the stock HDD with a 1TB SSD and left the 128GB SSD in place (it came with Fusion Drive)