As for this moment, there's no possibility to add curved walls directly.
1. Make a curved wall, simply splitting wall into small pieces and connecting them:
- click on a wall;
- menu will pop up;
- choose -.- icon.
(as in attached pictures)
2. Create small wall pieces with Magic Cube and connect them:
For a better understanding, I am sharing link to the project, made by one of the best designers, Jessica. Explore them and good luck in creating Your own!