How do I set up node pathing

Hello.

for the path finding in my project I would like to set roads that NPC's will walk on, But the player can avoid. I Could block off ether side of the path I want them to take. But I think it might be easier if I can somehow invert the cells. to instead of marking a cell impassable if a "Wall" is in it. to mark a passable if a "path object" is in it.

anyone know a way? or will I just need to do the first thing?

Thanks

I'm not sure to understand right but if you want NPC to use a fixed path, you can give a look at this :

Spline path (with checkpoints)