You can enter what ever character you want, it is the multiverse, as long as it follows the structure outlined for characters. 

I have added four starter worlds to the multiverse setting as well as the Nexus world. Clearly these worlds are meant to be used as a place for people to get started and begin their adventures in the NMV. 

You can create your own worlds, settings and places for people to explore, just add NMV at the end to identify them. You can create your own stellar empires that you head if you wish, but for ships and other sundry things such as key people in your world setting you will need to make a sheet for them.