I am looking for a way to edit individual mipmaps within a DDS file. To my knowledge, the NVidia Plugin only lets you create the largest image and then computes the smaller ones automatically. However, the problem is that after long tests with the different available algorithms and sharpening strengths I still think that at least the 2nd and 3rd of the smaller mips could look better if created manually. Is there a way to do so?
I have had some luck by using GIMP 2.8.2 and the DDS plugin for GIMP. It allows for opening, editing, and modifying the main image and the sub-levels, and then saving them. The levels are presented as layers in GIMP. I don't have any experience for the quality of the exports, as I've only actually had to use one level of mips in my own projects.
GIMP here: http://www.gimp.org/
DDs plugin: http://code.google.com/p/gimp-dds/
DDS support is standard in GIMP since version 2.10, plugin installation is no longer required.