@bitinn UE's navigation is good for the basics but it hasn't had much work done on it since UE4 was released.
Our navigation system (mercuna.com) is intended to cover many more use cases (and can handle spawning and building nav volumes at runtime, which I think is what you wanted here).
If this is all you need it maybe isn't worth it - you can probably find a workaround, but if you generally want more than Unreal can offer it might be worth a look!