Why would you want to do this? If you have sentimental data on an old handset, where there are no connection capabilities, then the only option would be to manually type the text into to a given thread. You would need to supply the date, time, message and whether sent or received. This is a nuisance I know but the only way to get sentimental data off such a handset. It can then be incorporated into the main thread as if exported. Perhaps only then can I finally get rid rid of my old Nokia 5310!