[Plugin: Promotion Slider] can a post be added to promotion page?

Using posts as the source for the slider will cause the links for the slider to point to the associated posts. Of course, promotions can be linked to anywhere.

Are you trying to display posts and promotions in the same slider?

Having a single slider (shortcode) that pulls both promotions and posts is a bit complicated. You would have to pass your own custom query results using the ‘promoslider_custom_query_results’ filter as defined here.

Here is some sample code that I used on a site to display specific custom graffiti jewelry products from the WP E-commerce plugin:

add_filter('promoslider_custom_query_results', 'tagurslf_product_query', 10, 2);
function tagurslf_product_query( $results, $id ){
    if( $id == 'homepage_slider' ){
        $results = array();
        $posts = array(
            4162, // Letters
            5847, // Love Pendant
            102,  // Graffiti Bracelets
            50,   // Rings
            111,  // Custom Pendant
            4279, // Graffiti Scroll Earrrings
            5852, // QT
            5862, // OM Pendant
            5890, // Dog Tag
        );
        foreach( $posts as $post ){
            $results[] = get_post($post);
        }

    }
    return $results;
}

Hopefully that will give you an idea. You could also run get_posts() for the latest post and then run get_posts() to fetch the promotions from a specific category and then splice them together.

Sure: http://www.orderofbusiness.net/request-quote/