Combine animations dynamically


we have a human arm model that we want to animate depending on input.
The animations should be proportional and simultaneous. That means we want to set an animation speed and we want to combine different animations, e.g. flexing the arm and opening the fist.
The hand model is quite complex. Is it possible to pre-define different animations for the atomic movements and let the engine combine?
How can we implement that in an easy way?