It’s actually the asset registry that you’d need to add the path to:
FAssetRegistryModule& AssetRegistryModule = FModuleManager::LoadModuleChecked<FAssetRegistryModule>("AssetRegistry");
AssetRegistryModule.Get().AddPath(YourPath);
That said, if you create a new folder* on disk it should just appear in the Content Browser.
*Caveat: Currently you’ll also need to create an asset within the new folder, as the Asset Registry directory watcher doesn’t watch for directory changes. Someone is working on fixing this for 4.22.