I had a similar problem on a site of mine (which at the moment isn't on line).
I solved it using multiple installations of the TinyD module, which is a version of Tiny Content which can be installed up to 10 times on a XOOPS Site.
Each installation of TinyD automatically gave me an entry in the main XOOPS menu and most of those installations had only one page in them. But for each TinyD module you can build a hierarchy of sub-menu categories.
Before trying to multiply install TinyD you should read the instructions (if I remember they're in the ReadMe file which comes with TinyD). Then you'll have no problem getting the menu structure you describe.
Good Luck,
Alan