Adding an index to a baseline FSM table

Hi Experts,

What is the proper process for adding an index to a baseline FSM table?  For example, we may need to add an index on the User_def2 field on the table, ATTACHMENT.  

1. What is the process?  
2. Do we add the index ourselves with the knowledge that any FSM updates/upgrades in the future would most likely remove this index?  Or do we submit a ticket to CS to have them add the index?

Thanks in Advance !