Top folders (root nodes in the navigation pane) are store. If you need to add a new store in the profile you can use the AddStoreEx method of the Namesapace class which adds a Personal Folders file (.pst) in the specified format to the current profile. See How to: Add or Remove a Store for more information.
In case if you need to create a top-level folder (at the same level with standard folders like Inbox and etc.) you can get the Parent folder object of the Inbox or any other default folder and add a new folder there. For example:
Dim objFolder As Outlook.MAPIFolder
Dim parentFolder as Outlook.MAPIFolder
Dim objOutlook As Outlook._Application
objOutlook = New Outlook.Application()
myNamespace = objOutlook.GetNamespace("MAPI")
objFolder = myNamespace.GetDefaultFolder(olFolderInbox)
parentFolder = objFolder.Parent
parentFolder.Folders.Add("Some folder", Outlook.OlDefaultFolders.olFolderInbox)
Also you may find the GetRootFolder method of the Store class helpful. It returns a Folder object representing the root-level folder of the Store. You can use the GetRootFolder method to enumerate the subfolders of the root folder of the Store. Unlike NameSpace.Folders which contains all folders for all stores in the current profile, Store.GetRootFolder.Folders allows you to enumerate all folders for a given Store object in the current profile.
Sub EnumerateFoldersInStores()
Dim colStores As Outlook.Stores
Dim oStore As Outlook.Store
Dim oRoot As Outlook.Folder
On Error Resume Next
Set colStores = Application.Session.Stores
For Each oStore In colStores
Set oRoot = oStore.GetRootFolder
Debug.Print (oRoot.FolderPath)
EnumerateFolders oRoot
Next
End Sub
Private Sub EnumerateFolders(ByVal oFolder As Outlook.Folder)
Dim folders As Outlook.folders
Dim Folder As Outlook.Folder
Dim foldercount As Integer
On Error Resume Next
Set folders = oFolder.folders
foldercount = folders.Count
'Check if there are any folders below oFolder
If foldercount Then
For Each Folder In folders
Debug.Print (Folder.FolderPath)
EnumerateFolders Folder
Next
End If