[In Star Trek Fluxx, Ensign Smith has the same function as the Expendable Crewman, and in Star Trek: TNG Fluxx, it’s Tasha Yar]
A: It all depends on who has the Expendable Crewman, and who played Exchange Keepers.
See this question in a video!
Our Friend The Expendable Crewman, Part 1
If the player who has the Expendable Crewman is the one who plays Exchange Keepers, they can choose to trade whatever they want. It’s only when someone else is taking a Keeper away from you that they must take/destroy the Expendable Crewman.
If, on the other hand, your opponent has the Expendable Crewman, and you play Exchange Keepers, you’ll be getting that dork in the red shirt, no matter what you’re actually coveting in your opponent’s collection.
Think of it this way: he readily obeys his own captain’s orders, but he always seems to leap forward at the last minute when a hostile force demands hostages!