Yes,  SSD's can be added as an upgrade but there is a high cost associated with the services that are REQUIRED for this sort of upgrade.  It is much more cost effective to build the cluster with SSD's from the beginning, as the majority of workloads benefit from the accelerated metadata anyways.
I have a customer with a X400 36TB Nodes and they are wanting to know if they can add SSD's for metadata acceleration?
Can SSD's be added to an existing Isilon cluster?