You can create an army, can't you? First make the army, select it from the factions tree and you can make the army's deployment zone (The depl. zone button get colored). Once you have placed it you can place the units (units bttn get coloured), assuming you have still the army selected.

Still dunno so much about the unit placement bit, I have accidentally wiped out entire armies. The editor is sometimes a bit awkward.

And if you find a way to add a working fort...