You can change a post to a page from the database, but there might be a field or two not set properly. I'm not sure though that's quite the functionality you are really looking for.
You could probably write them as normal posts, use a custom/meta field to identify them as side-menued posts, and then a quick query of the database to retrieve the list of posts with your special meta field set. This is similar to how people have done 'asides' or 'miniblogs', except you'd KEEP the tagged posts in your normal blog listings/archives/etc., and only show the title (and no content) in the sidebar.
In fact, this would make for a good plugin... a particular named meta field set to whatever values you want, and then be able to output a list based on a value. For instance, have the meta field be 'side_list', and the value be 'poem', for all poems you want to show up. Of course, if you want to be able to 'tag' things as poems, but ALSO decide to have some show on the sidebar, and some not, you'd ideally want two variables, one being the 'post type' (I have this concept on my site...), the other being the 'show on sidebar' flag.
If you give me more ideas, I could potentially flesh this out further for you (maybe even end up building a plugin).