Do you mean Mooncrash? That's the only expansion I know of for Prey. I'm not sure how it could use a different engine though.The Typhon Hunter extension used UE4. It looks like it's a separate game, though it requires Prey. The reception wasn't good at all: 'Mostly negative' on Steam.
*Edit* Ah ok, I see Typhon hunter is the separate online component of Mooncrash. I've never tried, but, judging by the reviews, it doesn't seem like I'm missing much.