One question that a situation like this raises is whether or not those static pages are complete pages (full html head/body with css and js file references) or if they’re “body only”.
The type of integration you describe is relatively easy if they’re page fragments. You can use them as data in the body in a template being rendered. (I’ve done that a couple times with Jekyll-generated pages.)
However, if they are complete pages with their own infrastructure, then it’s going to be easiest to serve them outside Django. This does not mean that you are unable to restrict access to them using your Django infrastructure. In this latter case, you would set these files up in a manner similar to your static files. (See the remark at Show external folder html content in django - #2 by KenWhitesell for some ideas in that area.)