Games : How to make User Objects for Universe Sandbox 2?

I've looked all over the Internet for a tutorial, but came up with zero results.

In the "Add" menu of Universe Sandbox 2, there is a button named "User Objects".

This obviously means there is a way to add your own objects into the game. What I don't know however, is how to do this.

There is a tab next to it called "Objects" containing things like a Dice and a Baseball.

How would I go about making my own?

Many thanks.