In this case you should manually edit wp-content/themes/woopress/header.php. We strongly recommend to make all changes within the child theme. You may read here about child theme: http://codex.wordpress.org/Child_Themes.
Please tell us what is the function of this button.
Unfortunately there is no feature to create a separate menu for fixed header.
As Brian said you need edit header.php and specify what menu to load in the fixed header. A new menu should be created in Appearance > Menu.