A number of carriers used to convert their day pax jets into night cargo jets. Eastern had pretty good results with their 727-100s with this as they had, I think 9 pallets pre-built passenger seats. UPS tried charter passenger service which went by the wayside as quickly as it started.
Not only do you incur an expense for the conversion, but there would be a loss of useful load, and space for overhead storage, and passenger service units, and additional lavatories, a cold galley area, added zone temperature system, just to name a few items.