hmmm, that's kinda hard...
first of all: your textures - did you simply make them out of photos or did you paint them pixel by pixel?
if you painted them it would be of unvaluable advantage if you have saved them as a layer-based format like .psd (photoshop) or .cpt (photopaint) and from there exported them into .tga. if you have a layer with your camo in the psd file it's quite easy to replace the original camo pattern and add another one.
ok... i assume the worst now in thinking you didn't paint every light n shadow yourself but made it all out of photos.
and i assume that you have a programe that supports layers like photoshop, photopaint, gimp or paintshop pro (in any case: ?(yourprogramme == ms-paint) : goto "exit" ;D ).
you CAN add another camo pattern, BUT you have to remove the old one without loosing all the light and shadow of the old one, so this requires a lot of image editing. i would lower the saturation of the area the camo pattern shall be added to first. a result would be a black and white area. now you have to delete the light/shadow differences that are caused by the different base colours of the camo pattern... do not remove the light/shadow that are caused by 3d effects like scratches in the metal, recesses, etc. that will be the most difficult part. if you've done that add a layer (yes... work with layers from now on!!!) containing the new camo pattern, but only to the area that you distracted the saturation before. choose 'multipy' or ' image overlay' or something similar for how the layer shall appear before it's background. fiddle about with the brightness/saturation/contrast, etc of the camo layer to fit the right colors.
do not merge the layer with the background but save it as psd ot cpt or whatever (a layer based format anyway) and export the image to .tga. then it should be pretty easy to convert it to pac or paa (use feersum.endjinn's paatool, not texview).
pheeew.... that's all i can reply to such a general question