Well, adding new employees is easy. Simply run an append query and, assuming that you have a primary key assigned in the destination table, the duplicates will be "kicked out."
I guess that to start with for the updates, you will need to have a way to know which records have been updated (a flag field or updated date field). Otherwise, you'd have to compare every field in every record, right? Not every record in the updating table will contain updated information, correct?