Can I add an SSD and keep the HD?

Your only option in installing an internal SSD is installing the Apple custom blade SSD. Here's the IFIXIT guide: iMac Intel 27" EMC 2639 SSD Replacement.

As you can see this is not an easy job! In addition locating the required SSD will be tough and costly! Frankly, I wouldn't do it! As you also run the risk of damaging the display assembly just in the process of opening the system which would be a very expensive mistake.

OK what to do??

The best option is to use an external SSD RAID drive set. In fact you'll get more performance than what the internal drive can offer. Here it the unit we use: OWC - ThunderBay 4 mini or the smaller OWC - Elite Pro Dual. Depending on the needed size it can also be cheaper!