If you need to add a single wall in your room – you can easily do that.
Just follow these steps:
Pick the room that will have an additional wall in the middle.
Decide from which wall that partition will start.
Pick the wall and split it into four pieces.
Drag the central piece in the middle of the room to the point where the partition ends.
Now drag the two outer corners to each other opposite to the partition end.
Now you can enlarge or move the wall at any angle.
You will need to drag the endpoint back to the wall and merge walls into one piece to remove that partition.