Program Files/steam/steamapps/common/GalCiv3/Data/English/Test (that's not exact wording but you should be able to find your way).
There is a file 'StarNamesText. Make a copy of it (rename it something like StarNamesText_OLD) and move it elsewhere (if you leave it there the game will add it in and complain about all the duplicate names, and you might want to go back to the original one day. Also keep a copy of your edited file... everytime StarDock updates GC3 it will over-write your file.
Edit the copy to have the names you like. Boot up the game and - voila - stars named as you like.
PM me with an email address if you'd like a copy of mine - it's mostly classical names.