Wordpress - Select subscriber as author of post in admin panel?

This is a simple hack I wrote in a similar situation. It will display all the Subscribers in the Author dropdown on edit/add post/page, from where you can select any one you want. I think it should work for you...

add_filter('wp_dropdown_users', 'MySwitchUser');
function MySwitchUser($output)
{

    //global $post is available here, hence you can check for the post type here
    $users = get_users('role=subscriber');

    $output = "<select id=\"post_author_override\" name=\"post_author_override\" class=\"\">";

    //Leave the admin in the list
    $output .= "<option value=\"1\">Admin</option>";
    foreach($users as $user)
    {
        $sel = ($post->post_author == $user->ID)?"selected='selected'":'';
        $output .= '<option value="'.$user->ID.'"'.$sel.'>'.$user->user_login.'</option>';
    }
    $output .= "</select>";

    return $output;
}

The trick behind this is, after you submit submit this page, WP only reads the $user->ID from this drop down in the $_POST array, and assigns it as the posts author. And that's what you want!

As of WordPress 4.4.0 you can now use the wp_dropdown_users_args filter. The code is much simpler now:

add_filter( 'wp_dropdown_users_args', 'add_subscribers_to_dropdown', 10, 2 );
function add_subscribers_to_dropdown( $query_args, $r ) {

    $query_args['who'] = '';
    return $query_args;

}

This is similar approach to @brasofilo. But only works in the edit post screen, rather than quick edit, and includes all users (not just authors and subscribers).

/* Remove Author meta box from post editing */
function wpse50827_author_metabox_remove() {
    remove_meta_box('authordiv', 'post', 'normal');
}
add_action('admin_menu', 'wpse50827_author_metabox_remove');


/* Replace with custom Author meta box */
function wpse39084_custom_author_metabox() {  
    add_meta_box( 'authordiv', __('Author'), 'wpse39084_custom_author_metabox_insdes','post');  
 } 
add_action( 'add_meta_boxes', 'wpse39084_custom_author_metabox');  


/* Include all users in post author dropdown*/
/* Mimics the default metabox http://core.trac.wordpress.org/browser/trunk/wp-admin/includes/meta-boxes.php#L514 */
function wpse39084_custom_author_metabox_insdes() {
      global $user_ID;
      global $post;
      ?>
      <label class="screen-reader-text" for="post_author_override"><?php _e('Author'); ?></label>

      <?php
        wp_dropdown_users( array(
             'name' => 'post_author_override',
             'selected' => empty($post->ID) ? $user_ID : $post->post_author,
             'include_selected' => true
        ) );
}

This mimics the default author metabox but the call wp_dropdown_users omits the who=>'editors' argument. It defaults to the only other value which is call users.