It can be done in different ways, but one easy way would be:
You could create a new table "TableA" in database with only 1 column called "LastExportedEventLogID".
When you are exporting the event logs update TableA with the Last Event log ID.
And when you start exporting the event logs next time you know which once to send as you have got the last exported event log ID.
By doing this you are not modifying any of the kentico tables.