This in fact' 'is'' possible. There are 3 SATA Data Connections on the motherboard. See image here. There are also 3 Distinct Power connections on the motherboard shown in the image. In order to retrofit a SSD into the new 2011 21.5" iMacs, You'll need the following parts: 539-1295 (SSD DATA) 539-1296 (SSD POWER) And the brackets, which part numbers have not been identified for yet. See image here for an idea of the fit.
There are Three SATA Ports