Here is a function that allows you to add new ones dynamically. It also calls the corresponding preprocess function if you have one declared.
Then call drush cache-clear theme-registry
to get it working.
To use it replace THEME with your theme's name, and place it in your themes template.php file.
e.g. For a theme named Droid you would call it droid_preprocess_node(&$variables, $hook) {
...
function THEME_preprocess_node(&$variables, $hook) {
$view_mode = $variables['view_mode'];
$content_type = $variables['type'];
$variables['theme_hook_suggestions'][] = 'node__' . $view_mode;
$variables['theme_hook_suggestions'][] = 'node__' . $view_mode . '_' . $content_type;
$view_mode_preprocess = 'THEME_preprocess_node_' . $view_mode . '_' . $content_type;
if (function_exists($view_mode_preprocess)) {
$view_mode_preprocess($variables, $hook);
}
$view_mode_preprocess = 'THEME_preprocess_node_' . $view_mode;
if (function_exists($view_mode_preprocess)) {
$view_mode_preprocess($variables, $hook);
}
}