Correct me if I’m wrong, but I don’t think FBX exports IKs, or at least UE4 doesn’t import them.
The animation is baked on export by Maya or Blender.
The clean solution is to just animate with your tool of choice.
Make a small loop with only the arms of the character positioned like you want them to, for each class of weapon.
Then you can just blend that animation in with the walk cycle/whatever and you’re good to go.