How to create a submenu to a registered category with xdg-desktop-menu?

[Frédéric Mercille]

I know I can use xdg-desktop-menu to add an application to a "registered 
category" (e.g. Office) in the desktop menu with a *.desktop file, and I can also 
add a new submenu at the root of the desktop menu with a *.directory file. What 
I would like to do, however, is create a submenu in a submenu that is already 
present.

Here's an example of what I would like to have :

(MENU)
  |
  +-- Games
  +-- Graphics
  +-- [...]
  |
  +-- Office
  |     |
  |     +-- MyVendorName
  |     |       |
  |     |       +-- MyAppName1
  |     |       +-- MyAppName2
  |     |       +-- MyAppName3
  |     |
  |     +-- OpenOffice.org
  |     +-- Evolution
  |     +-- Kontact
  |     +-- [...]
  |
  +-- Settings
  +-- System


Using xdg-desktop-menu install MyVendorName.directory MyAppName.desktop 
creates a submenu at the root of the menu (sibbling of Games, Graphics, Office, 
etc.), but I don't want to pollute the root with my apps which clearly belong 
under the Office category (and the Category key is ignored for an entry of 
Type=Directory). Does xdg-desktop-menu allow that?

Thanks.