I didn't try to do this in the demo Version but I have heard the only thing you can do with new graphics in the demo is to replace existing graphics maybe even using the same names. I recomend being very careful changing things in the demo because the game won't run if some files are changed. Its a good reason to get the full Version just about everything can be tweeked. There are however some extra neutral empires that can be added to the demo you can find them on
http://www.se4hub.com/universalshipyards/ They work in the demo and full Version. Some error Messages do come up when starting a game with them because some of the fleet files and so forth are missing but they work anyway and will just use the default values. If you add the correct files to them the error Messages wont come up. Just copy the files from one of the other neutral races if you want. Its good practice for starting to modify race files to find the missing data in one of these files and correct them.