fist you need to install a plugin that lets you edit DDS images in Photoshop or install a image tool that can edit DDS image files
then you can use 7z to open the textures.jar file, Note i said open not unzip
Once open find the texture you want to edit and double click the image, it should open inside photoshop, where you can edit it and then just click save
Which files exactly are you talking about? I know Photoshop can for instance open and save DDS files with a plugin.
Adding more clothing items (or any other kind of items) is not possible at the time, the only thing you could do is changing the textures of existing clothing items. To do that extract the texture you want to edit from the jar file, edit it and then repack it in the jar file replacing the old one.