Update Historical Items

Question

I have created a simple survey list which has already generated some data. Now we need to extend it with some new columns. One of these columns (a person lookup) will need to have its data populated for all existing records and it is fine just to copy the item creator value in it. All I have access to is the survey/list settings (full control privilege) as it is a sharepoint service we are provided with by a 3rd party. Manually updating each record is not an option due to the amount of historical data, so we need to do some sort of batch processing. I was thinking creating a site workflow to run it against this list would probably do but how to do that exactly if feasible? or is there another option for me? Thanks a lot in advance