Help with Animations

warning I'm pretty new to unity! so I have a walk animation using a blend tree but i cant play another animation on top of the blend tree. I want to have a different arm idle animation when holding a sword. but i don't have any kind of body rig, my player model is made up of 3D objects. So I was wondering How i could play both at the same time.

 

You create another layer just for the arms in your animator:
https://docs.unity3d.com/Manual/AnimationLayers.html
And assign a avatar mask to it. You can create avatar masks by right clicking in the project view then Create/Avatar Mask.
Then you can mix them together by adjusting the layer weight.