What you’re describing is used often to adjust a shape of a part of the body when in a specific pose, this process is called corrective blend shapes, so that the part of the body when performing that specific pose ( usually based on the joint angle ) has a tweaked shape ( bulk of the shoulder, elbow skin compression, and so on ).
I won’t raccomend to use blend shape to do what you described, simply because it would be faster and easier to add ( in Maya or whatever ) an additive animation layer, where you can add a bit more movement to the base mocap animation, because it allows you to still keep the original animation, but add something a bit different on top of it.