That's right. Currently, you can't create a many -> one definition in the mix dictionary.
But you can apply the mix duration when you call SetAnimation.
Like Nate mentioned, but in C#, it would be:
TrackEntry entry = skeletonAnimation.AnimationState.SetAnimation(0, "damage", false); // returns a TrackEntry reference.
entry.MixDuration = 0.25f; // Set the MixDuration property.