Hi, that's a good question.
Off course, you should find in the ClusterXL R77 Admin Guide every parts of the big picture you need.
Waiting such documentation, I can propose you following steps (for High Availability or Load Sharing modes):
> force fail-over to the functional Security Gateway
> then: just prepare the new Security Gateway restoring every configuration needed
> redo the SIC
> ensure that you've configured ClusterXL as "Maintaining current active Cluster Member"
> install policy and ensure you synchronization is working using "Making Sure that a Cluster is Working" chapter of the admin guide
> test forcing fail-over o the new node
Hope that'll help.
Information Security enthusiast, CISSP, CCSP