Integrating other pages in WordPress

Hello all. First I’d like to say that the support forums have so far been very helpful, and I’m very happy using WordPress.
My dilemma is this: I use wordpress in the root of my site for the majority of my content. I would like to add additional content to the site (which are in subfolders) that displays in the section where the posts usually show up to keep the site look same-same throughout. The content to be included would be just regular ol’ html. One of the reasons for this is if I need to change the location of one of my sections, I’d like to only have to edit the menu once.
I have tried using iframes, but have a personal dislike for frames.
What I would like to do, just because I think it would be cool is to use index.php?section to link to the section, displaying the contents in the ‘content’ section of the page.
For reference my site is https://www.salens.org and that is where wp is installed. Some of the other content is https://www.salens.org/projects/ which isn’t a full representation of what that section will hold, but it’s a start.
Thanks in advance for any help that can be provided, and I’ll understand if it just cannot be done.