You're correct that right now, the plugin can only create individual posts. Can you tell me more about what you mean by "assign these posts to a particular page" -- are you talking about adding each email to one Page (as in, a page created under Pages->Add New on the WordPress Admin dashboard), or about having them all show up on a single sub-page on your site (like a category or tag archive page)?
You can accomplish the latter by assigning a category or tag to the posts using shortcodes. You could even customize your theme not to show those posts on the front page.
The former is a lot more complicated and I'm not sure how it would work, but I'm open to suggestions. Would each new email's contents get appended to the existing page? Before or after the existing content? How would you tell WordPress which page you wanted it to appear on?