Can I add an HDD to an SSD only iMac?

Well you do have some work here. As this system is so new I doubt many people have even opened one up yet. Luckily IFIXIT has! In reviewing the late '13 teardown you can see the layout and on the most part the internals are very similar to the late '12 model.

Given the fact you got a SSD only model I don't know if the HD SATA cable present or not, if it is you should have an easier time putting in the HD.

Here' the IFIXIT guide to replace the HD on the '12 model iMac Intel 27" EMC 2546 Hard Drive Replacement. If it is missing you'll need to follow this guide to put a new one in iMac Intel 27" EMC 2546 SATA Cable Replacement.

Please tell us what you find if the SATA cable is missing or not.