You can create any sort of custom skin you like and fly it offline but the game will not let you use it in any online arena, even as a skin that only you will see. For online use you have to submit your skin to HTC for approval. As Vraciu says in order for it to be approved it has to be of a scheme that was historically used in WW2. Proof that your aircraft existed (i.e. a photo of the real plane) will be required as well.
