Hello csperling87, Your MD3220i supports both SAS & SATA drives.  You are correct in that you will need an interposer on a SATA drives.  Also, you will need to make sure that the drives are on the support matrix with the correct Dell part#.  Here is the link to the support matrix & if you look on page 23-30 it list out all the supported drives as well as the Dell part# for the drives.  [URL] Please let us know if you have any other questions.
I currently have a MD3220i that is set up for SAS.  My vendor sent Dell branded SATA SSD and didn't realize we were on an SAS backplane.  When installing the drives I get the error: "Bypassed Physical Disk". I've read about needing an interposer, is this required? I've also read about "ATA Translator Firmware".  I'd like to if possible just get these working instead of having to return/get SAS drives.  Any ideas to get these working?   Just an FYI, found this:
MD3220i Mix SAS and SATA