Why can't you resave these as TGO's with your edits?
Because they often use the shader array and no parts? The object isn't part of a population or anything.
Note: If you're wondering what the heck I mean by Path Tracing Ready; Every part of the model has some level of default shader reflection, with roughness based on grayscale inversions of the diffuse, adjusted. Much better results than before. This looks good in PT, but can look too shiny in normal renderer, at least the falloffs are harder.