I'm not sure about the n800, I have the n810. I know it's possible on the n800 but it is supposed to be more complicated. The easiest way would be to copy the navit.xml file to a computer, edit it and then copy it back.

I did ask one of the developers if there was a plan to add an interface for changing settings. He said it is planned but it didn't seem like it would be anytime soon.

Personally I think Navit is getting stable enough and usable enough to be worth the setup pain. However many of the features that make it user friendly and a complete mapping solution are likely going to take some time before they show up.