An addition to the polygonal models tools.
Paste and weld pre fabricated meshes to selected polygons.
Ideally, it would be best to sculpt the mesh first retpologize and then add finer details. Finer details should be left out of the retopology process especially if they might be repetitive in form are may not respond well to auto retopology. (which would force the user to do this process manually)
The best part about is that this feature wouldn't be particularly hard to implement. (unless of course we were to account for a topology mismatch.)
Here is a video demonstration of what I mean.