If an excel source is already cataloged and profiled and a user wishes to add new columns, he can add, catalog and profile them individually and it will not have an impact on profiling of old columns or affect any rules created on source previously.
From Administration tab, configure Excel source and add the desired column and click on Finish. This will catalog new column.
Now when you go to profiling details of this source, you will see this column. You can individually profile it and create rules on it. This will not affect your previous rules.
Note: Merge Catalog feature is available for both EXCEL and CSV.