The appliance that is being added, needs to be in a factory fresh state. If it is already or previously configured, you cannot add it to an existing cluster.
In order to make the "new" nodes factory fresh, you will need to run a reset. There are procedures in the VxRail SolVe desktop. If you are not an EMC employee or Service Enabled Partner, please reach out to your local Dell EMC VxRail account team for further assistance as these procedures are restricted.
Hope this helps