Unfortunately, that's quite impossible with how the engine works. Npcs are one big mesh/texture that includes both clothing and body. It needs to be this way so that the engine can load many of them at once. Even if you could find a way to mod in npc customization (which is highly unlikely anyway,) it would take up a huge amount of video memory. Even coop needs 1Gb of vram to work correctly which is due to having 2 players at once both with customization options.