This is not a feature in osTicket.
You would need to write a script to take your old tickets and import them into the database directly.
Or It might be easier to have it use the tickets api to open a ticket, and then edit the database to change the ost_ticket.created, closed, lastupdate, updated dates. Now that I think about it you would also need to update the dates on the ost_thread_entry for the tickets also.