A : Using this plugin with existing sites with video is extremely easy and intuitive because it will find all existing videos and add them to your sitemap the first time that it gets generated, provided they're added to Wordpress in a way the SVS can find them - which is likely. So if you have a blog with a 100 post and 100 youtube videos or vimeo videos, you'll end up with a video sitemap with 100 entries generated in just a few seconds after installing the plugin.
For self-hosted videos, you'll need to add them wtih the shortcode button (takes a few seconds, also has an upload feature as well) so that the plugin knows what to look for to find them and it will embed and play them in JW video player, so it's actually multi-function in that way.