https://buddypress.org/forums/topic.php?id=1314 start there. Look at the code for bp-core-adminbar.php
Create a function that is your new top level item and submenus. Use add_action() to hook up the new function/menu item.
add_action( 'bp_adminbar_menus', 'bp_adminbar_custom_menu', 40 );
function bp_adminbar_custom_menu() {
?>
<li>
<a>"<?php _e( 'Custom Pulldown', 'buddypress' ) ?></a>
<ul>List Item</ul>
</li>
<?php
}
Remember to include the content as a list item! <li>
or it will not appear fine 🙂
Nicola