Pages from a site cannot immediately be simultaneously used in an app as well. The reason for this is because generally Pages are implemented differently for an app vs. a site. If you still really want to have your site simply wrapped in an app you could possibly employ your own custom copy mechanism that takes pages from a site and copies them to an app.
Keep in mind however that assets can be shared across a site and an app. This includes images, videos and even text with the new content fragment feature in AEM 6.2.