Character animations - Seperating a body parts animations/splitting animation

I am about to start animating a character for use in a unity game, but I am running into a problem:

I need the have animations for him to run like normal, and also need to be able to have his right arm holding a weapon, and aiming the weapon with LookAt towards a raycast (his firing of the weapon depends on a ray from player input depending on platform).

So the problem means I need a way to have his arm as either a seperate object inside unity, and handle the aiming seperately? or perhaps I need to make changes to his skeleton in Blender to allow that arm to be seperately animated? How would you accomplish this kind of animation system with blender/unity?

Keep in mind that the character is either standing still, or most of the time, running forwards, and that he never rotates his body, just aims his weapons around as he fires toward targets.

Look into AddMixingTransform function.

See this tutorial: http://unity3d.com/support/resources/example-projects/3rdpersonshooter
And video “Unite 09 - Character Animation tips and tricks” about it here http://unity3d.com/unite/archive/2009

Thanks for the help, what worked for me was to animate everything but the bone i needed to script the movement for, then drag and drop in inspector, and modify the transform of the bones game object, how easy! I love unity lol.

Anyway thumbs up to You guys for the pointers!