Well, I dunno about photoshop, matey - but in Paintshop it goes like this:
1. save as TGA in texview
2. open TGA_picture in Paintshop
3. click "Load From Alpha Channel" in the MASKS menu.
4. Then in SELECTIONS, click "select all"
5. Now, copy & paste "...as new layer"
6. Goto LAYERS and click "delete" ( = delete the orig. alpha channel )
(
insert yer own re-texturing process here)
7. When yer textures are done, click "save to alpha channel" in the MASKS menu.
8. Save the TGA, close the whole crap, then re-open it & close it again!
9. Now, you should be able to open the TGA with texview & convert it to PAA/PAC.
In short - screw photoshop and use PS instead - it often comes in comp'zine CD's (free trial).
Laterz!