Even though Dell says it's not supported you totally can, you will just need the new backplane(KVGG1) and SAS cables(TK2VY).  You'll basically just pull out the plastic stoppers that are covering the last 4 slots, replace the backplane and cable and you should be pretty much good to go.
I have a R620 (Personal information removed by Mod.) with a 4 bay backplane. I want to upgrade to an 8 bay backplane. How can I tell if an 8 bay backplane is compatible and will work with my server? The 8 bay backplane am looking at is "Dell KVGG1 PowerEdge 2.5" 8-Bay Backplane with Cables 0KVGG1" 
Dell R620 backplane upgrade help.