The short answer: Yes!
The long answer:
We have set the goal that anything you were able to do in Version 2 will also be present in Version 3.
However, we will abandon the "handcdrafted xml" paradigm completely.
That means, in Ubox3, you will be able to not only create complex Simulations directly in the application, without ever touching an XML file, but also create completely new objects using a range of editing tools and wizards.
We're focusing heavily on procedural content, so you will have the option to create all kinds of celestial objects by simply tweaking a set of visual parameters, rather than tinkering with photoshop.
Additionally, since we have to depict known objects as good as possible, you will have the ability to use custom texture maps and even custom meshes at some point.
But we will most likely encourage you to use the procedural approach, since it's a LOT easier and faster to create something really cool. And it makes sharing objects and simulation so much more convenient, since you don't have to send megabytes of images over the wire.
I hope this answers your question.