When we imported records from a previous system into Totara Version 12 we used a Type Category so all imported records came through coded to show they were imports from XYZ System. The records were able to be imported with both completion and expiry date.
You just need to make sure you have got the appropriate field names to match the import.
I would suggest testing it with a small number of records as if there is an error you can't bulk delete the uploaded records and if you do it again you will end up with duplicate records as it doesn't replace the previous one (my learn learnt!).
Sue