Check this site for possibilities. (If you don't mind a bit of hacking)
http://gpsunderground.com/forum/
I've updated my portable GPS map files several times over the last few years using it. In fact, it's running a completely different operating system than it came with using info I got from this site.
Don't know if there is anything on there specifically for Nissan nav systems. I just bought a new 2011 SV (no nav) for the wife yesterday and registered on this forum last night.
So I'm just now starting my trip down the Pathfinder path. Looks like this forum may be as good as the one I refer to for my Miata (2003 Shinsen Version)
Hope this helps.