Create gallery with images on hard drive plus some already in media library

Hello
I’m setting up a page that consists mostly of photos. I wanted to use the ‘gallery’ to publish a set of images in a neat grid in a post/page, and have Easy Fancybox (plugin) do it’s magic when displaying them full size. Now I want to add a gallery to a page (existing, but empty of content), using images on my hard drive waiting to be uploaded, plus some of the ones that are already in the media library. This seems not to work smoothly..

If I upload the images on my disk, they automatically become the gallery for that page. If I save that and save the page, and then try to add to that gallery images that are already in the media library, there just doesn’t seem to be an option to do that. Or am I missing something?

Or do I need another plugin for this?

Any advice would be greatly appreciated

Thorir V