You can add items to NPCs to carry by editing the WORLDMAP.TXT file, but I'm not too sure about how the trader inventory works, if it's connected with that. I think it has to do with script files, since I'm using a mod by Endocore that effects that and it's a script file. WORLDMAP.TXT is pretty easy to edit since it's notepad, but script files are a different thing. I don't know how to edit scripts, but there are a few people around here that can.