Enlarge/extend exisiting navigation mesh

[Dim67]

I want to enlarge the navigation mesh of a cow barn. I could only find a handful of YT tutorials on how to CREATE a new navigation mesh, but I want to enlarge an existing one and I cant get it to work. I can draw a new plane to the size I want and create a new nav mesh from that, but now I have 2 navmeshes so obviously that doesnt work.
Any help would be appreciated.

[Eische]

Extending is not possible.
Sometimes the planes are still there, only hidden. So you can delete the nav mesh, unhide the old planes, add new ones and create nav mesh again.