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.
Is possible, here's the complete guide: