You could actually do something similar by using a model with the third person avatar and just the arms showing (hide the rest or delete mesh). For example use FPS Mesh tool. But in all honesty the project is designed in a professional manner, therefore the arms are separate for FPS view for a reason. You simply will not get a desired effect using humanoid for first person in my experience, therefore my advice is to bite the bullet and do it rightWhen I am playing in first-person mode, can I use third-person arms for all animations, not the first-person components and they are invisible? How?
How are you attaching the object? A brief description of how you are attaching would help to answer your question.I used humanoid animation in first person arms, they are moving as animations but the item which they are holding is not moving with arm. Why? How to fix this?