Very simple. Log in to your existing fabric interconnect currently running the domain. Type "connect local-mgmt". In this next command the IP address is the virtual IP for the cluster. You will use this going forward to use UCSM. Type " cluster enable IPAddress". Answer yes to the prompt. Now cable your second interconnect to the L1 and L2 ports like a normal cluster setup.
You will need to rework your service profiles since you now have a B side available.
http://www.cisco.com/c/en/us/td/docs/unified_computing/ucs/sw/gui/config/guide/2-2/b_UCSM_GUI_Configuration_Guide_2_2/b_UCSM_GUI_Configuration_Guide_2_2_chapter_0101.html#task_9858FADCDF6E4D2B852118CCEEBAA3DC