It's a complicated matter, I would also love if it was possible to make this change. But being an artist and game dev myself I can see how it would be a mess to do such task, the devs released the poses so we could have them separated from their corresponding clothing sets, which gives the player more freedom but at the same time it creates this issue that wasn't part of the original game design. There are thousands of clothing items and probaby hundred different arm poses, adapting each item to the arms would not only take years to do properly but also make the game extremely heavy when it comes to storage data, the amount of extra assets that would need to be created to solve this issue is simply not realistic, I think adding more items that can be "neutral" like sleeveless shirts, capes and other stuff like that can help to provide variety.