When on-boarding a new subsidiary we add accumulated history data for all migrated projects on the timesheet of a Dummy User. The only limitation we encountered was that a single entry cannot have more then 1000 hours on it. Our work around was to create different task time types and booking 1k per task-type/day/task combination. You could also put it on different days or different tasks.
Our record timesheet has 92k hours on it ;-)