You can, I was saying that the manny hands have both the left and right hand bones inside of them. Which I assume was Epics way of making it easier to animate them since the same animation blueprint can be assigned to both.
The animation blueprint can be assigned to any mesh that uses the manny skeleton.
As for where to insert your custom posing, I would put it into the DefaultPose pin of the blend node:
That is the animation state when the hand is not actively being driven by hand tracking or gripping.
Or add it to the blend prior to it where it checks for the hand tracking and replace the idle animation: