hello;
to resume, you just want to edit the textures without to touch the model, and recompile it, right ?

so you will have to find the VMT(materials) and VTF file (texture) in the GCF files ,
get gcfscape before, and install it.
https://developer.valvesoftware.com/wiki/GCFScape

you just have to be capable to open the VTF file into a 2d software like photoshop or paintshoppro. there is a plugin for to export in VTF format or you can use VTF edit. but you have to create your texture in TGA format file first in photoshop.
vtf Edit:
https://developer.valvesoftware.com/wiki/VTFEdit

and the photoshop plugin :
https://developer.valvesoftware.com/wiki/Photoshop_VTF_Plugin
i think psp is able to use photoshop native plugin.

when done, your texture must replace the original files, but don't delete them , just rename the materials file, and edit the vmt fwith notepad and give the path of your new texture.

//red