Sorry No, you can't have more than two SATA devices in your system.
But you might be able to gain some improvement by using a hybrid drive (a single device that has a HD with a SSD cache). Seagate makes a nice one they call a SSHD. Here's more on it Seagate SSHD. Follow this IFIXIT guide on how to swap out your drive iMac Intel 21.5" EMC 2389 Hard Drive Replacement
But before we can go there we first need to see what HD you currently have in your system. Go to About This Mac > More Info... > System Report and look under SATA tell us what you have now. Why this is important is you normally need to switch out the same manufacture of HD. If you don't have a Seagate drive you may need to replace the thermal sensor cable as well.
OK, so lets say you go with this SSHD what to do with your current HD and you stated you wanted to still have Windows BootCamp. You should be able to use your drive internally if you want to give up your Optical drive following this IFIXIT guide Installing iMac Intel 21.5" EMC 2389 Dual Hard Drive
So with this setup you gain almost what you want. - Good Luck!