Blend animations for specific bones/body regions

Hello all,

I have recently started working with ThreeJS and I have a very specific scenario which I have not yet come across on the internet. Perhaps you experts could share your insights.

I am working on an application to allow the user to pose a patient on a surgical table.
This includes several base poses, like: lying on stomach or back, and several additional poses, like: arms besides body, left arm extended, legs bend, etc.
All my positions are basically an animation clip of x frames where the armature does not move.

I want to have a dropdown-menu for the base poses, and for the arms and legs region. To do this I need to be able to blend animation clips for that specific region. (legs, arms) with the base pose.
How could I go about doing this? Is it possible to apply a pose/animation clip for specific bones?
Please let me know if my question is unclear, I can elaborate.

kind regards,