What the page was built with is irrelevant. wpgraft reads the page as your browser rendered it: the real elements and the real computed styles on each one, at every width you ask for. It then builds that as a Shopify section made of theme blocks, with every style in one of the blocks’ own settings. Nothing that arrives depends on the original site still being there.
The general case for a store: a landing page, an about page or a lookbook you like the look of, wherever it lives, written into your Shopify theme as a wpgraft section. Every box, text and image in it is a theme block with its own settings, so the copy is edited in the theme editor like the rest of the store, not pasted into one block of custom code.