but really don't know what to do to make new body presets.
The items in the new body preset list is hard coded. It's not currently setup to allow the user to make additions or modifications.
The actual data values come from the Appearance.xml, LocalObject.xml, Stars.xml, and Galaxies.xml files in the 'Data' folder where the program is installed.
Note that while you're free to edit these files, any changes will be overwritten (without warning) when a new update is installed.