When you add a new animation to a character in Spine and want to use it in Unity, you typically need to export the updated skeleton data from Spine and replace the existing files in your Unity project. This process involves replacing the .json or .skel.bytes files with the new ones. After importing the new data, you may need to set the mix duration for the new animation if it requires specific blending with other animations. This can be done in the SkeletonDataAsset Inspector in Unity, where you can specify the MixDuration for each pair of animations. Unfortunately, there isn't a more streamlined solution for this process, as it involves updating the animation data and ensuring the correct blending settings in Unity.
