How do I stamp nothing?
Short answer would probably be no to the actual question. That being said it should be a fairly simple thing to fix.

1) Have your process bulk insert the CSV file to the same table every night after a TRUNCATE. 

2) Do a LEFT JOIN to find with a null against your employee table to find the new ones and insert them.

3) Do a right join with null against your employee table to find the "users no longer on the feed" and update their "left_date" field.

If you need to run these updates manually (sounds like it from your question) you could still have the SQL just create the statements and so you could review and run them manually.