You can create a room continuing to draw an existing wall (see the following image) until you create a closed profile.
Select the command New wall, place the cursor at the end of the existing wall, and when the edge of the former wall turns highlighted, click the left mouse button and keep drawing the room. Once you close the shape, the Application automatically converts the new polygon to a room.
This article is valid for DomuS3D® 2020 and later versions