Put main-level pages in sub-navigation

I’m looking for a way to custom-pick the sub-navigation items, regardless of the parent/child attributes. I’m using WP as a CMS and I have about 50 pages. I want to make them all top-level pages (no children) for several reasons.

Sub-navigation, as far as I can tell, is completely determined by the page structure (parent/child attributes). Is there another way to build a sub-menu under the main links on the navigation bar?

For example, on the navigation bar I have About Us, Products, Partners (etc.). I’d like “Partner Solutions” to appear in a submenu under both Products and Partners. Further, when my boss changes (again!), I want to be able to put Partners under About Us (for example) without making it a child and changing the URL structure!

I can get closer to this functionality by giving up on pages altogether and using posts (not pages) with categories in the navigation bar. However, then “category” appears in the URLs, I must still designate sub-categories, and giving up on pages is undesirable for this CMS-style site.

Does my question make sense? Does anyone have any ideas?