The field would be nonspecific to records, so all records would have the same text.
That is the definition of a global field – or more precisely, a field with global storage.
I do this several ways in my solution.
1) as erolst suggested, create a global field and use a set field step in the startup script for the file to put text into it.
2) if the text is specific to the record (or records) exported, then you can define a text field with auto enter option and insert data or a calculated field. Be aware that this will only fill in new records not existing records and that you will have an extra field which can be handled another way.
3) (my favorite) script the export with a button and use the set field script step to insert the appropriate text into the global field. This option allows you to export field or meta data (i.e. current time or found count) as part of the text.
Hope this helps.