I assume there is only 1 "date row", and it's always the first row of the file ?
If so, you should extract this date in an ODI Variable.
Then ignore this first row in your ODI interface. But use the variable to populate your "insert_date" column.
If you can't directly read the file to extract this row, create a specific interface that will load your flat file with the filter ROWNUM=1 (filter on staging area zone). This interface will only extract the date in a temporary table. You can use the "yellow interface" for that.
Then, an ODI variable will read the date in the "yellow datastore table".