David,

No, you can't join two ROOMS.
Yes, you can join two ROOMS.

In v2.6 you can't join two rooms, because in SH3D a "room" consists of a floor and a ceiling, without the walls surrounding them. If you install the Advanced Editing plug-in you can change the form of a room very precisely in any way you may want, by deleting or adding control points ("corners").

Hatching patters do not appear in rooms, but in WALLS. You cannot join two walls in SH3D. However, if you install the Advanced Edit plug-in, you can join or connect two walls, so they are one.

Read the plug-in tutorial to learn more.

In order to draw very large rooms, you can click the "-" magnifying glass in the menu bar. You can then zoom in and place corners more exactly by dragging them, or by entering exact coordinates.

Hans