As per policy, direct crossovers of franchises are not allowed. This was set by community consensus many moons ago and has survived at least one concerted effort that I'm aware of.
If you are taking a person from one franchise and altering such a key element of their character as their species, then you're not exactly importing a direct copy, are you? In either case, it might be better to come up with an original character to write about that might draw inspiration from the crossover character.