Welcome to the Forum!
To be honest the JDM sat nav is pretty useless and cannot be converted. If you are lucky you may find a DVD unit on ebay from a trashed UK car which can be swapped over and used with your existing screen etc.
Another option is to PM nurrish as he fits mini car PCs (and is about half the price of the nissan units) with a touchscreen SatNav, Movie playback, mp3 playback and about anything you can do with a PC