It's not a design I'd advise, but if that's what you have to work with then the concept would be to :
- Import data into an intermediate table (can be created on the fly if preferred, or re-use the same one over again).
- Run an update query on your main table with an INNER JOIN betwen matching records (You don't indicate what a match is so you'll have to manage those details yourself).
- Run an append query where the key of the intermediate table is LEFT JOINed to that of the main table, and the WHERE clause only allows those records with a Null key in the main table to be processed.
- Clear down the intermediate table for next time. This will mean either clearing all the records or deleting the table, depending on your choice in step 1 above.