Add a feed with all posts unread

When I discover a new blog I often want to read/skim the older posts. If there are many posts this takes a long time and I’d love to be able to use newsbluar to keep track of how far through I am. The problem is that when I add it to newsblur only the recent posts are unread. Even if I wanted to manually mark each post as unread, I can only do that on posts from the last month or so.

Is there any way to add a feed and have all of the posts unread? Failing that, do you have any suggestions for how I might solve my problem by using newsblur differently?