Chosen Solution
My Mac dealer told me that I can only replace my HD with an SSD, because they don’t do it, if the iMac was not configured that way, when I bought it, as a there is a connection missing on the main board. Is it possible to have both HD and SSD or do I have to give up my SuperDrive and replace with an SSD?
Yes! Here is the IFIXIT guide: Installing iMac Intel 27" EMC 2429 Dual Hard Drive Kit. In your case its going to be a 2.5" SSD