Can I have Post Types under other Post Types in Admin menu?

Yes, this capability is available with register_post_type, via the show_in_menu argument, but whether or not the particular plugin you are using supports this I don’t know.

add_action( 'init', 'wpa70679_custom_types' );

function wpa70679_custom_types() {

    register_post_type( 'parent_type',
        array(
            'public' => true,
            'labels' => array(
                'name' => 'Parent post type'
            )
        )
    );

    register_post_type( 'child_type',
        array(
            'public' => true,
            'show_in_menu' => 'edit.php?post_type=parent_type',
            'labels' => array(
                'name' => 'Child post type'
            )
        )
    );

}