After integration SunBlogNuke with Window Live Writer, you may wonder where to find or what to replace the excerpt field in the built-in edit post panel. There is one elegant workaround – using the <!--more--> tag to mark the end of the excerpt. Fortunately, there is a setting property called "supportsExtendedEntries" in the wlwmanifest.xml depending on the feature. Please note that setting this value to true will allow the <!--more--> tag in your Window Live Writer. 

What you need to accomplish is placing your cursor where you want the “Read More” tag and selecting “Split Post” from the “Format” menu. Then the collection page and simple entry partial will be modified to have a "Read More" link whenever there is extended content. Have  a look at the screenshot as follows:

extended-content

Also you can see the more features integrated with WLW, for example, keyword, category and slug etc. :)