customize the menu of magazine theme

The Magazine theme says that it doesn't support custom menus but I want to customize the menu. I have two issues with the automatically generated menu: I want to add a link to an outside url and I want to add posts without having them show up in the menu.
Basically, the only way I can see to add pictures to the main page slide show is to create posts but I don't want the posts to show up in the menu. Also, the site is a sub site of a multi-site installation and I want to add a link back to the home site on the main menu. Please advise as to how I can do these two things to my menu.