This may need a nicer interface in the future... seems legitimate.
But right now, in code, what you do is immediately set the MixDuration for the animation you queued.
For example:
skeletonAnimation.AnimationState.SetAnimation(0, "jump", false);
skeletonAnimation.AnimationState.AddEmptyAnimation(0, 0, 0);
var track = skeletonAnimation.AnimationState.AddAnimation(0, "death", false, 0);
track.MixDuration = 0f; // 0 seconds of mix from empty to "death"
The shortcut would be
skeletonAnimation.AnimationState.AddAnimation(0, "death", false, 0).MixDuration = 0f;