Not to be a self-promoting twat but look at steam guides. Although is far from easy or simple and even being gentle to your poor computer, the guide will show you how to make a custom party of four. You will be using Lan connection which does not actually require a real internet connection, and once characters made, save the game on the host profile and close down the other clients!. and you only require one client to play/control all custom characters once character creation is completed.
I seek to find an easier way but I still hope the game devs provide the means through UI I even believe it already has been coded just waiting for release.
http://steamcommunity.com/sharedfiles/filedetails/?id=769494798
