This tutorial may provide some answers for you:
SolPart Menu UncoveredYou can have several Solpart menus in your skin, just use the tokens like this:
[MENU]
[MENU:1]
[MENU:2]
If you set a page so it is just available to registered users, and then add all further pages that are available to registered users as sub pages of this new page, you maybe able to configure it how you wish using the above tutorial.
If I was using the HouseMenu it would be easier, where you add [HOUSEMENU] to the page and you can configure it to only display the menu items from a tabid, so you could specify to just display the tab id of the main menu item for the registered users and all of its sub pages.
Hope this gets you started,
Thanks,