I have tested this, and NO, it is not possible to load streaming sublevel of another streaming level. You can LoadStreamLevel level with sublevels though, but sublevels will not be loaded.
You have to OpenLevel with sublevels, to make sublevels openable, i.e. level with sublevels must not be sublevel of another level.
PS: the same is true for WorldCompositions, i.e. WorldComposition cannot be sublevel of another level, i.e. it can, but sublevels of WorldComposition will not be loaded, like with regular streaming levels.