I think it's not possible at the moment, but you can use something like that:

fun <T:Any> wrapAsExpression(query: Query) = object : Expression<T>() {
    override fun toSQL(queryBuilder: QueryBuilder): String = "(" + query.prepareSQL(queryBuilder) + ")"
}

You have to write for your query:

val rating = wrapWithExpression(RaitingsTable.slice(listOf(RatingsTable.rating.max())).select { RatingsTable.id eq ItemsTable.id }).alias("rating)
ItemsTable.slice(ItemsTable.col1, rating).selectAll().orderBy(rating)