[Plugin: WordPress SEO by Yoast] Automatically Generate Keywords

Hello, I’m currently creating posts through a form plugin, allowing users to submit and create posts at the front end. However, this also means that the posts will not have tags. I would like to take the Generate Keywords feature from the plugin and hook it to the form plugin so that it can automatically generate keywords and save them as tags.

The question is, how can I trigger the Generate Keywords feature outside the post editor and can the function generate keyword based on the post id or the content?

http://wordpress.org/extend/plugins/wordpress-seo/