Ive got an idea running around in my head. In the fluff you tend to read about races the Imperium is fighting with (and/or exterminates) alien races of various shapes and sizes. I thought it would be a fun thing to do a campaign on the war(crusade) against one such Xenos races On one side youd have the Imperium and on the other side youd have the various other factions. Either joining in for kicks or for their own nafarious reason.
There comes the problem though... There are only rules and ship stats for excisting races.... SOOOOOO how would i represent those alien races?