It is though it can be really problematic because of ambient occlusion which is part of a skin. Like if you move a decal then you also move AO on the decal, with .psd files they are different layers but with existing skins they are the same texture. Are you new to skinning? You need image editing software with .dds plugin, then open the .dds file and edit it. Though make a copy first and don't edit the original, at least not without making a backup. If you have no experience of skinning then I'd suggest learning to make skins from scratch first, helps you understand what to do and what not to do.
It's also possible to make changes to skins without editing the textures, simply by combining and renaming existing files. For example you can add new colors to plain colored cars, usually the color file is named metal_detail.dds. You can change rim color, interior color, create new combinations from existing files. Content Manager is hugely useful in this since with that you can easily find out which textures affect which part of which car.