Making a corridor is actually a very simple task when you get down to it.

Create a room by cutting 3 256x256x256 cubes out in a straight line, then create another room parallel to this but 512 away.

Set the textures to whatever you want but set the middle wall on the facing sides to the archway texture Door2B in the Ancient package.

You should end up with something looking like the picutre on the right.

Then load the 2D editor.

To set the texture as the background choose File->Image->Get From Current Texture. This will load the texture into the background of the editor.

The simplest way to create the cut out section for the door is then to move the 4 corners of the square to the sides of the black area and make top segment a bezier curve. If you then move the bezier points to just above it creating a lovely curve you should end up with something looking like that in picture 2. The green marker is the pivot point of the created shape which can be moved and the red marker is the centre of the shape and cannot be moved.

You can increase the smoothness of the top line by going to Edit->Segment->Detail and choosing say 10 (I think the default is about 4).

Once you are happy with the shape click on the Extrude icon or choose Process->Extrude and enter 512 for the depth of the corridor.

You should then be able to move the shape between the 2 archway textures and subtract, creating your corridor. The final product should look something like that on the left.