Hello IPM, Yes you can add another MD3200 to your R610 server & get both to work.  Since you are running out of space you can also just add an MD1200 if just needing space as the MD1200 is cheaper than a MD3200.  The MD1200 will connect via sas from your MD3200 & can provide you extra disk space if that is all you are looking for. Please let us know if you have any other questions.
We have the SQL cluster with two nodes (both PowerEdge R610 Rack Server) connected to the DELL PowerVault MD3200. As we are running out of space the idea came to add additional DELL PowerVault MD3200 to the cluster. Is it possible?
Is it possible to connect two DELL PowerVault MD3200 to the single PowerEdge 
R610 Rack Server