You should be able to import multiple entries in the same file.  The first column is defaulted to be the date of the entry but you need to use the number of distributions column to tell it the import when the journal entry stops.  So if you were importing two different entries on two different dates and the first entry had 10 rows and the second entry had 5 you would have the number 10 in the # of dist column in the first 10 rows and the # 5 in the next 5 rows.  Another option would be to export the general journal for a month out to Excel and see what it looks like and try to duplicate that file layout with your actual import.