Sadly Nokia does not allow any 3th party app access to theis offline Nokia maps.
How ever CacheMe supports offline maps. You can create them with http://mobac.sourceforge.net/ , i.e. from OpenStreet map.
Make sure you save the map as "Big Planet Tracks SQLite" file. Then store it somewhere on the phone and load it the CacheMe maps settings.
Works quite well for me. Together with offline GPX files (Generated as premium member) I am completelly able to use it without internet connection. Note: even use use the offline map and GPX files, CacheMe will try to go into the internet ats startup. Simple deny it and it will work fine.