There are a few threads that have been created over the course of last year til present. The core point is what makes business sense and what doesn’t - at the moment what you’re after just isn’t viable (unless there has been a change in the first 1/4 of 2021)
As you likely know already, you unfortunately you can’t ‘top up’ orders unless you add the single items at the ‘Exclusive basket offers’ sections before checkout, or you add it to your subscription(s). As an alternative, you could pop to your nearest Sainsburys to grab your extra Chocolate and Vanilla RTD!! 
drools at sight of fresh RTD
In the below thread there are more links to past threads related to this matter. Feel free to indulge 