Hey,
Sorry for the delay in getting back to you. This might be a bit unconventional and I haven’t tried this yet but what if you add the source files where it should be and just exclude them for ignore file for Perforce? You would need to make sure non modifies the engine source files but that would mean the structure means the same and keeps USG happy?
Perforce itself doesn’t care about the Unreal Engine source files as its just a revision version control system.
Another thing I just remember, if your “unreal engine source” folder is 120GB, you are adding all build artifacts and files that should never be tracked in source control.
For the engine source, you don’t need to track:
/Engine/Build/*
/Engine/Binaries/*
/Engine/Intermediate/*
/Engine/Saved/*