Hey MM,
I assume you mean take the html from a standard webpage and place it into the WP "Pages".
I'm just digging into Thrive stuff myself and I tried it just the other day. The page I was working with had the CSS in the head so I tried using Thrives custom css option.
I believe you'd also have to upload images to the media library and redo all the src's accordingly ... which I did.
Not sure what I did wrong but it didn't work.
I'm not giving up on it and will try to remember to come back if/when I do figure it out.
As a workaround I did as Al suggested and made separate directory/folder for each item. If you decide to go this way remember to make a subdomain. WP will not acknowledge a directory/folder setup outside of WP.
In my case I made a subdomain "specials" them place each item under it in it's own directory/folder.
Hope this helps.