Can't import a standalone animation clip into Umotion

I wanted to base my UMotion animations on the idle pose of my character.

UMotion starts animations from a T POSE (or from the default pose of a model I presume)

However that seems unnatural to me and I was hoping to start them from the idle pose clip I use for my character.


But there's just no way to force UMotion to load that clip.
I do the Import Clips dialog and I get this error:

Is there no way to start the animation from an idle animation? The bone positions/rotations are quite different than in T POSE, so starting it from idle would achieve a smoother, cleaner effect. I saw the import animation tutorial but I think it imports an FBX file there, meanwhile Im importing a standalone animation clip (*.asset extension).