Irregardless of what any temp rules say, if it's decided by the person creating the challenge and they say that one can import objects, and then the artist imports the main object of the scene instead of making it in Bryce, well then it just dosen't end up being a Bryce challenge anymore.

Small imported objects that just add to the scene are perfectly reasonable. But this should be stated clearly in the temp rules for each challenge.

If the person decides that any imported objects are acceptable including anything that will play a large part in the scene (the theme for instance), well that cannot then be a Bryce challenge.