I am not sure that adding new VMs is the best way to go if you want to expand the cluster. There are a number of port forwarding settings in the sandbox that may create problems if you attempt to go this route. If you wanted to build out a full cluster, my suggestion would be to download the Ambari installation and go that route preventing you from experiencing issues. The sandbox is meant to provide a high level glimpse into Hadoop and the Hortonworks distribution in general. I hope this helps.