mixing two animations in one Rig

Firstly sorry if the Title is confusing, because I'm a programmer and I'm so new to skeletal animation.

I'm using a model with 2 animations(my artist is still refining them): 1. Idle 2. Walk (lower and upper body are animated) 3. Pickup Object (lower and upper body are animated) 4. Throw (only upper body animated)

What I want to do?

I want to make the character walk, pickup an object from the ground and lastly, throw it, using their respective animations. As of now I'm only testing the animations and I don't pickup or throw anything.

The Problem

When I move, the animation work fine. For the other two animations I cant blend walk to them. For example, when the throw animation is playing, if I start moving, the throw switches to walk(both arms come down as per Walk).

Question

How do I blend the animations i.e. pickup while walking and throw while walking. It would be helpful if you suggest something done through unity itself(probably scripting or somewhere in the editor). My artist is busy and I can't ask him for too many animations