can we archive the import file in any way?

As far I can tell only the import error response csv files are kept in the import log for 2-4 weeks.

It's a security risk for NetSuite as there can be sensitive data imported - there is no functionality for handling this for you. However - you can do a saved CSV import from the file cabinet with script - so if you did it that way - you'd wind up with an archive in the account - but it wouldn't be very... manageable