How are you importing the spreadsheets? A for each file loop? If so you would have a variable to switch out the connection string to the spreadsheet.

I would create a simple stored proc to update the table set the "file detail" column equal the a variable where the file detail column is null, then add in an extra execute sql task to execute the proc and pass in the variable from the package.

Or if your comfortable with coding you could always look at creating a .NET script like in the following article http://www.select-sql.com/mssql/how-to-use-a-variable-inside-sql-in-ssis-data-flow-tasks.html