You may want to upvote the idea here: https://community.tableau.com/ideas/8719
This is relatively simple to do with Tableau Desktop, so I am a bit surprised its not as simple in Prep -- however, I would continue to expect updates as time goes on (there is an update nearly every month). The product was released without cost, and I believe that is because they wanted to release knowing that there are missing features they still want to add. Just my guess.
This was now released in Tableau 2018.3.1