Building navigation paths (not obstacles) at runtime


I’m making a management sim game, let’s say it’s a Theme Park clone. I want all of the AI to move along paths that the player can draw.
Now I assumed that they way to go abotu this would be to make a placeable path actor, and have that set the navigation bounds. I want the nav mesh to only include the paths that get drawn a, so if there are no paths drawn, there is no navigation.

All the documentation I can find talks about how you can create a nav mesh and the use actors to block cretain parts of it, but I want the actors to actual form that nav mesh.
Is this possible? Or perhaps there is a better way to go about this?