Hi,
Static NavMesh does support streaming. You will need to place at least one NavMeshBounds in Persistent Level and other volumes in sub-levels. Volumes in sub-levels will cut part of navmesh and save that part in sub-level. At runtime when sub-level is loaded/unloaded it will dynamically attach/detach navmesh part to a main navmesh in P-level.
NavMeshBounds in P-level is required to let navigation system know that it should not delete main navmesh actor in P-Level even when all sub-levels are unloaded.