I've never been abducted in TS4 or made an alien as they don't really interest me, but some NPCs are aliens in disguise. I recently discovered this by accident when I took a townie into CAS for a makeover (the getups some sims choose, good plum man!). If your legacy rules are that you can only marry NPCs, I suggest taking the townies into CAS when they spawn in, just to check. If you don't alter them, I personally don't think of it as cheating, but everyone has different opinions on that ;) Either way, hope you find your ET!