I think the only place the "read more" link is used is in the blog feature. It sounds like you want to create an HTML instance with one or more instances that link to other page(s) with the full text.
If I understand correctly, this is something you'll need to do manually. Create the full text page(s) first, and note the URLs for them. You can keep them from appearing on the menu by unchecking the "Include in Menu" checkbox on the page settings.
Next, Create your HTML feature instance on the main page. Include the excerpt and a (manual) link to the full page.
Hope this wasn't too remedial of a response!