'includes' in wordpress

I installed wp3 in my root directory where my existing site is and have made them ‘play well’ together. My existing site is html with quite a few includes… specifically the navigation (main menu)

I can’t do ‘includes’ within a ‘text’ widget. I’d like to pull in my menu via an include within a widget. That way any sub-site use the main menu without having to code anything.

Editing individual themes or building (recreating) the menu with a ‘menu widget’ is tedious & unsustainable as more sub-sites are added

Any recommendations/advise