You can not merge two clusters without losing data. If you have only external shelves, you *may* be able to cable the shelves to existing heads, import the aggregates and data, and then add the heads to the cluster separately and then recable the shelves back to the original heads. I haven't done enough research to see if this will work since I've never had to do this. You will probably want to contract with NetApp Professional Services before attempting something this complex.