I know is a very late reply but if you still need the code here it is. If you don’t need it any more hopefully it can help others.

function change_album_gallery_link( $permalink, $post ){
if( empty( $post->post_type ) || $post->post_type != 'ims_gallery' )
return $permalink;
global $wp_rewrite;
if( $wp_rewrite->using_permalinks() )
return $permalink . "/slideshow/";
else  return $permalink . "&imspage=slideshow";
}add_filter( 'post_type_link', 'change_album_gallery_link', 50, 2 );