You can add nodes, but you cannot merge two clusters and keep data and SVM of both. Nodes will need to be wiped clean, so any data will be lost. To preserve data it has to migrated to new cluster first.
After validation, if you can mix your controllers, you should upgrade your old cluster to 8.3 (to have both in same Data ONTP version) and then join the new and clean hardware to the existing cluster, if you already have data en the new cluster you will need to move your data, wipe the cluster and then join the new hardware to the existing cluster. And you can use your existing switches CN1610 (remember that you need 2) for intercluster conectivity if you will have a 4 nodes cluster.
If you upgrade both clusters to 8.3.1 and have the space, you can use SVM DR to migrate the SVMs from the older cluster over (including the configuration, not just the data). Then you could wipe out the old cluster and join the controllers as nodes in the newer cluster and rebalance the load.