add a new class style to the dashboard dropdown

I'm working in the blog module on business catalyst. I need to add some class styles to the drop down menu so the BC users in my group who are not code writers can apply a styles not provided on the 0verall template. To be more specific I need to add an image resize class and a caption class to the dashboard "Apply CSS Class" dropdown when in the Blog Post, New Post dashbord. From what I can tell the class styles are generated by the server in a file named ModuleStyleSheets.css. When I open that file I'm warned not to add anything to it.

/* For backwards compatibility reasons, a link to this .css is generated automatically in rendered pages. Please keep this empty file in order to avoid 404 W3C validation errors. */

My question is, How do I add new class styles to the dropdown without using the ModuleStyleSheets.css?