First, let's get a little more detail, shall we?
How complicated are these web pages? It is simple informational pages with just text, or are you adding graphics, forms, tables, etc. The more complicated the page you are having the user create, the more work there is for you to do. The easiest way around this is to have prebuilt templates, where the user fills in the information.
Second, adding items to a menu dynamically is feasible, but your design considerations will determine how difficult a job this will be. Optimally, back-ending your menu structure into a relational database would be ideal. You could add, edit or remove menus, submenus, menu items, etc with relative ease with a system like that.
If you don't have a back-end relational database, you could create a series of delimited text files and use them like database tables. Of course, that means you would have to create your own database routines to access those files. That adds a huge layer of complexity to the process, but could be doable in a pinch.
Finally, you could use a DHTML menu system that's already out there and use PHP to manage the menu item includes. I don't want to even think of the complexity of that!
How's that for a starting point?
