Follow these instructions at your own risk. I recommend you back stuff up.

This information is for people who want to make their own patches or tweaks.

Tools:
GIMP - https://www.gimp.org/
GIMP DDS plugin - https://code.google.com/archive/p/gimp-dds/downloads
BIMP - https://alessandrofrancesconi.it/projects/bimp/
GIMP Normal Map plugin - https://code.google.com/archive/p/gimp-normalmap/downloads
ImageMagick - https://imagemagick.org/script/download.php

I will assume the above are properly installed.

For small numbers of files, you can load up each dds texture into GIMP, and generate a normal map by going to Filters > Map > Normalmap.

I used the following settings:
filter = 4 sample,
minimum z = 0,
scale = 10.0,
height source = average RGB,
alpha channel = unchanged,
conversion = None/Normalize only,
DU/DV map = None,
and all options unticked.

Apply the map and export your texture to dds with the proper name. That means that "file.dds" should be saved as "file_n.dds".


To do this for a large number of files, I had to get creative because BIMP wouldn't handle the dds files properly. Wizards smarter than I probably have a better way around this, but here's what I did:

Batch rename all the dds extensions to .ico. I did this with the powershell command:
Dir *.dds | rename-item -newname {  $_.name  -replace ".dds",".ico"  }

Now BIMP will work; set it up to apply the normal mask and add the "_n" to the files. You can try to tell it to change the extension back to dds, but it didn't work right for me. Instead, I used imagemagick to fix it with the following command:
magick.exe' mogrify -format dds *.ico.

Clear out the ico files and put the files in your textures path or package for installation with a mod manager.