On Taxonomy Template page, want to add Post_Type

You can use pre_get_posts filter hook to check if you are on the taxonomy page and if so just add your custom post type to the query something like this:

function add_my_type( $query ) {
    if ( $query->is_tax('YOUR_TAXONOMY') ) {
        $query->set( 'post_type', array('post','YOUR_CUSTOM_POST') );
    }
}
add_filter( 'pre_get_posts', 'add_my_type' );