Edit family -- add geometry(void)---load into project. workflow?

Hi everyone,
I have many generic families in my project ( they are only in project not saved separately in local) among which many of them intersect each other so I want a create a void on one of the two pairs in intersecting families.

I have arrived at the geometry to subtract from one of the two elements but, could some one help me on how to create a void inside a generic model family and load it again into the project.

Here are the files thanks,
wall intersection on generic model.rvt (3.3 MB)
delete walls.dyn (38.0 KB)