[dba-SQLServer] Syntax to add field to index

I need to create an index on a table, then use the index to update another field, then add that 
other field to the index.  Is there a syntax to add a field to an existing index or is it just drop 
/ create index?

I create indexes in stored procedures, but I have never added another field to an existing index.

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John W. Colby
www.ColbyConsulting.com