I have something that works, but cuts the headlines by number of characters. You could do the same with words, but I need a set amount of characters for my site. This works for shortcode, not sure about the rest…
In your back-end, go to: Plugins > EDITOR
Select the plugin “RSS Multi Import”
I use the template “Simple Bullet List” so I edit “wp-rss-multi-importer/templates/simple_bullet_list.php” with the following:
$charlimit = “60”;
Then add substr() to the code. In my case, the output for headlines is:
$readable .= '<li class="title">'.substr($items["mytitle"], 0, $charlimit).' - <span class="date">'. date_i18n($strDate,$items["mystrdate"]).'</span>';