OK, just had a quick play, it's VERY easy to setup a new menu item...
The only challenge with this setup is you need to have the Documentaries folder included in your library, so it will actually show up in your TV Shows menu. This is not a limitation with the Skin, but a limitation with XBMC itself. There are workarounds for this, but essentially, it will involve you setting up additional profiles where you can setup different libraries, and basically spending too much time getting it working... (trust me, I looked into it).
The easiest way to avoid this is to create "Smart Playlists" for your TV Shows, Movies, Doco's, etc... (ie. anything in SMB://SERVER/TVSHOWS, SMB://SERVER/MOVIES, SMB://SERVER/DOCUMENTARIES respectively) You then need to add Custom VPLAYLIST items to the main menu for each of these (I've only done it with Doco's, Kids, Comedies and left these items in my main library, as I don't mind them being in both locations) and you're good to go.
To add VPLAYLISTs, you need to press 'down' twice to get to the settings menu, then go across to 'Customizations'. Once in the Customizations screen, press up so you can choose +VPLAYLIST. Select the Playlist or Smartplaylist you wish to add to the main menu (the the My Menu item, depending on preference) and then choose "Main Menu". This will add an items called Menu 1 (or Menu 2, 3, etc...) which you can then rename and sort. Select 'END' and you're back to your main menu with your additional items in it...
Nice and Easy
(I worked that out in about 5 minutes without instructions, so just have a play)...