Make a new layer, name it doorway.  With your selection tool set at rectangle, and feathering set at 0, antialias checked, make a tall rectangle.

Go to selections, modify, expand by 60 pixels

Now, click on your mover tool.  Place the mover tool in the selection, right click and move the selection where it looks good.

Flood fill the doorway layer with a darker green.  *Note, be sure that the doorway layer is on top.

Make another rectangular selection, go to selections, modify, expand by 60 pixels.  Click on your mover tool, right click on the selection and move it inside of the darker green doorway. When it's where you want it, flood fill with white.

Deselect, and click on your selection tool.  Select to trim the excess along the baseboard.

Hit delete

You can use the Retouch Tool/Soften to blend in the white area of the arch.

Ta Da! Now off to decorating :))