In Ghostwalk it says that humanoid souls are able to resist the pull of the "True Afterlife" and return to the material plane. These souls become ghosts and are forced to take the Eidolon class. Is there any way for a fey or outsider to become eligible for this mechanic? I'm running a Ghostwalk-based game and I want my players to have a larger choice of races than just humanoids.
If it's your game, simply say that all souls, not just humanoid ones, can resist the pull. You're the GM, you can ignore or change any rule that you want. Trying to find a RAW solution when a rule change is much easier is often less effective.
If you're looking for a RAW way to make it work, there's no way that I know of to make a fey or outsider into a humanoid.