If the query just returns one value, you can:
- Run a Execute SQL Task with your query, saving the result into a SSIS package variable.
- In the data flow task, read in the flat file, then add a Derived Column transformation, where the expression is simply your package variable.
- The data flow can now be directed into your table of choice, including this value.
You can also:
- Load the data first, leaving this field null.
- Execute a set-based UPDATE statement to update all records on the target table.
This, of course, assumes that your target table is empty.