Compute Shader PSO Dx12 I am new to the DX12 world and I am currently trying to tweak the nbody_gravity DX12 sample, but appending or calling another Compute Shader which is to add 2 buffers to produce an output in a different buffer. 

Right now, I have made a call to my ComputeBufferAddition shader and was wondering if I need a separate Compute PSO to do this, or can I reuse the one already present to call the NbodyGravity.hlsl
If I understand you correctly, you've added a new compute shader. In DX12, you'll need at least one separate PSO for each shader, since the shader code is part of the PSO.