Adding Html Page to Multisite

Hello..
I have a multisite setup showcasing themes. On each demo page I have installed a different theme mysite.com/demo1 mysite.com/demo2 etc.
I now have some other templates that I wish to showcase. ..mysite.com/demo3? They are not WP, just HTML templates.
My question is, am I able to incorporate these into my 'site' somehow. I am guessing that this is not really too 'Wordpress' involved, and that the answer is going to be that they are installed into my root folder somewhere seperately....so sorry if this question is way off beat!