Attaching a Custom Taxonomy to Posts?

You can use register_taxonomy_for_object_type function and if you say that the taxonomy is created by a plugin you can use the plugins_loaded hook, to make sure that taxonomy wast already registered so:

add_action('plugins_loaded','add_custom_taxonomy_to_posts');

function add_custom_taxonomy_to_posts(){
    register_taxonomy_for_object_type('TAXONOMY_NAME', 'post');
}