It's been a few years, but When I was doing texture pipeline stuff, we had a work flow like:
Photoshop > TGA > composite > DDS > custom format
We saved Targas from photoshop as our 'source' format , these were the 3DS Max textures
We had RGBA Diffuse, RGB Normal and Greyscale Specular TGAs
We scaled, filtered, and composited The Specular image into the Alpha channel of the Normal map using Imagemagick
The results were then mipped and converted to DDs with ATI' s The Compressonator utility.
(which some one here on TAO recommended to me)
The Compressonator looks like it has since moved to open source on git hub.
Image magick had DDS support , but The Compressonator offered easier/better mipping results.
for manual conversion , Nvidia has a Photoshop plugin..
Both Imagemagick and The Compressonator are command line utilitites.
I implemeted there use both via windows .bat files and by building command stings in Max Script executing with
DOSCommand . A similar approach with Python
subprocess.call() or similar ...python has a variety of ways to execute command line stuff.
If you are comfortable with Windows Batch scripting , .bat files are kind of cool because you can set them up to run on whatever is dragged and dropped onto them, or run on everything inside the same folder, and just hand them as little 'droplet' tools to your artists.
Caveat: I've been out of texture pipelines for a few years so better solutions may be out now days.