I was wondering if there was any way of importing a GeneratePress demo site design in that instance- or would I have to just convert to the standard GeneratePress theme and then Customize the look manually?
we don’t advise adding a Site to an already built website. Chances are things will break.
You could test it out on a cloned site. A clean install and import your content. Or just have a nose at the GP settings etc and apply them to your own.
Things to watch for, is the Site install will overide the menus / logos etc, replace any widgets and of course change the customizer settings. Also Sites have to use Additional CSS… so look their for any style conflicts.