The Imperium will tolerate some aliens who serve a purpose and are willing to be branded and are vouched for by a powerful patron such as a rogue trader or inquisitor. There was one official model released of such a sanctioned alien – Krashrak the Stalker. And an alien booster pack was also released with alternate heads and additional weapons.
Fortunately quite a number of alien creatures have been produced by other companies at 54mm.
I have another few aliens that i will eventually paint up.