iMac 21.5 inch Mid 2011 has an additional SSD BTO. You can have one 3.5 inch Hard drive, one optical drive, and one 2.5 inch SSD, however you can have an additional SSD!
Apple did not include a mounter for SSD with non-SSD model, so you cannot add an SSD since it has nowhere to mount. Supports SATA 3.0

Internal SSD installation kit for iMac 21.5 inch Mid 2011.
Package includes:
1. SSD brackets (2 pcs)
2. SSD mount screws (4 pcs)
3. Screws for brackets (3 pcs)
4. SATA data cable
5. SATA power cable
6. Installation Manual

Installation is not easy.  Expert skills are required. Manual is written for experts, so it is not step by step instructions. If you can disassemble an iMac completely, you can do it.