Importing FBX with actions in multiple files

I have a client that wants me to add a few additional actions to an existing model for a video game. The game is made in Unity and the model has each action in a separate file.


When I try to import any of the files I get this message:

Python: Traceback (most recent call last):
  File "C:\Program Files\Blender Foundation\Blender 3.0\3.0\scripts\addons\io_scene_fbx\__init__.py", line 216, in execute
    if import_fbx.load(self, context, filepath=path, **keywords) == {'FINISHED'}:
  File "C:\Program Files\Blender Foundation\Blender 3.0\3.0\scripts\addons\io_scene_fbx\import_fbx.py", line 2852, in load
    _(); del _
  File "C:\Program Files\Blender Foundation\Blender 3.0\3.0\scripts\addons\io_scene_fbx\import_fbx.py", line 2849, in _
    root_helper.link_hierarchy(fbx_tmpl, settings, scene)
  File "C:\Program Files\Blender Foundation\Blender 3.0\3.0\scripts\addons\io_scene_fbx\import_fbx.py", line 2344, in link_hierarchy
    child.link_hierarchy(fbx_tmpl, settings, scene)
  File "C:\Program Files\Blender Foundation\Blender 3.0\3.0\scripts\addons\io_scene_fbx\import_fbx.py", line 2300, in link_hierarchy
    (mmat, amat) = mesh.armature_setup[self]
KeyError: None

location: <unknown location>:-1

The mesh and armature are loaded but there is no animation.

I can load each file in Noesis and export it as GLTF, which loads fine in Blender. The issue is that this is a separate model with only one action. Is there a way to load the model with all actions, add additional actions, and export it with the same structure that it had before?