hey, im not an expert in skinning but in SA if u want to edit the dtx files within the rez files, there is only 1 way(for now). You have to download a modified gvi file to replace within the SA folder, so that next time when u launch SA, u force it to repatch itself. So during the *patching* process, when it says "uploading" you go to run->%temp%. Now,u have to find the hidden SA folder which just appeared and it is named by 8 random letters. After find the folder, all the SA_weapon or SA_tex stuff will be in dtx mode. There u can replace stuff or edit them with photoshop.
I know i suck at explaining sutff zzzz. btw I play SANA and weapon and map skins works.