If you ammend some of the config files you can extend your rank structure up to Vizeadmiral. If you're playing straight GWX the changes you need to make are simple cut and paste, if you're using SHC you still use the same data but need to add extra variables so that SHC will recognize them. This thread has the info on how to modify the config files to add the extended/higher ranks:
http://www.subsim.com/radioroom/showthread.php?t=174120