I recall it being possible to make an object a subpart of another object by dragging and dropping its entry in the item tree into the expanded list of subparts of another object. I think the object had to already have subparts for it to work, though. Also I haven't tried it in years, so I don't know if it STILL works.
I remember that it was definitely possible to save worlds as objects, though, by changing the extension from .a2w to the extension for Alice Models (which I have forgotten...
). There were a few weird side effects, but it basically let you turn any arrangement of objects into a collection that could be imported all at once.