Is it possible to make a folder look and behave like a file?
Apple Developer info says yes, you can definitely make your own package or bundle. Seems as if you want a package.

Package vs. bundle is discussed here.

Basically - register your package with the system, so that folders with the extension are displayed as a package rather than as a folder (application info.plist must appropriately set key LSTypeIsPackage for each of the package filetypes you need to create). Then set your app to automatically (using NSFileWrapper) or manually create the package itself on save.