The answer to this question is no, not without interrupting the running smartfail.  The reason for this is that adding a node changes the cluster group statement, which would cause the running flexprotect job (smartfail) to be system cancelled. The flexprotect job would then have to be restarted. 
Hello,  Does any of you know if a node can be joined to an Isilon cluster that already has a smartfail operation in progress?  We have to perform a test at a customer (on his behalf):      a) Remove a node      b) Some read/write tests      c) Add the node back  I want to know if we can process with adding the node back, before the smartfail process (from step a) has finished.  We are running latest 7.2.  Thanks  Francis
Add node to Islon cluster with smartfail already progress?