You can certainly have this setup. Although we are not integrated with WordPress directly (the Unbounce page can’t be hosted with your WordPress site,) you can certainly link to a WordPress page.
With Unbounce pages you can create button, image, or text links to different pages, whether it be another Unbounce page or an external webpage like the one on your WordPress site. So in this case, you will simply want to create a link that leads the customer to the digital download sales page on your WordPress site. The Unbounce page will act as a click-through page. And since Unbounce pages are not yet connected to any given transaction gateway, I would say that this is an ideal setup when using the landing pages with existing sales pages.
If you choose to go with this setup, you can use our external conversion tracking (http://support.unbounce.com/entries/3…) in order to track the full sales funnel.