How to add part of assembly to part in nested assembly? (See picture)

Hi,

I have multiple nested assemblies in my main assembly. I am trying to include a part in 1 of my assemblies when I extract however (the grey plate). I want to include this grey plate in my smaller nested assembly (colored blue).

Now, I know I can create a sub-assembly by right clicking 2 parts and pressing "add to sub-assembly." but I only want that plate (& not what it is attached to), which means I will have to slice it somehow...but I don't want it sliced in the main assembly.

Does anyone have any ideas?

See attached image for clarification

SolidworksModeling And Assemblies