Try the SR3 texture tools - if they won't work then you'll have to wait for someone to release something that will.
I have verified the SR3 texture tools I grabbed a while back work for SR4 assets, I've used them to unpack and repack a few normal maps.
For Spacecataz's convenience:
http://www.saintsrowmods.com/forum/threads/sr3-texture-utilities.566/
I'm not sure if that's what I have. It's been a while since I downloaded them.