Hi Greg,
Adding additional LIFs to a portset will not be disruptive. The igroup is already bound to the portset, the hosts are just waiting for you to open those LIFs on the other nodes so they can access the LUNs.
This will however add additional paths to the LUNs on the current nodes 1&2 - so need to ensure Selective LUN mapping (SLM ) is configured to ensure only the local nodes hosting the LUNs are available, or that the additional paths do not exceed any maximums for the OS type - see SAN Configuration Guide.
Hope this helps.
Cheers,
Grant.