There is two alternative ways to do this:
1. Add a column to the CSV file that has the title Closed and each line to be imported has the value of Yes, this can then be mapped to the closed field in the ''Sales Order Items'' folder during import.
2. Set the default value as Yes for the Closed Item line field in the mapping stage of the import.
1. Navigate to Setup> Import/Export> Import CSV Records
2. Import Type = Transaction
3. Record Type = Sales Order
4. Select CSV file to import > Next
5. Data Handling = Add > Next
6. Map fields in the file to the NetSuite fields
7. Sales Order Items folder> Closed: use the pencil Icon to set default to yes or map to the closed column from the Field Mapping section. > Next
8. Rename Import Map Name> Save and Run