In Kreo 2D Takeoff you are able to unite several selected polygons into a single polygon.
How to merge polygons?
Select several polygons on your drawing.
Right-click to enable the Context menu.
Click on the "Merge" button there.
Press the "Apply" button to save changes.
📝 Note!
If you would like to save the initial polygons, press the "Keep polygons" button.