How can i add a new mesh with new bones on a existing rig both in humanoid

Hello, first of all sorry for my english i do my best.
So im a programer who want to do an animation on my humanoid rig (i think its necessary for layer properly ?) in Blender, but i have also a gun mesh and it’s bones inside this rig for moving the mag and cie, so it work if i put it in Unity, but if i put the animation on a rig who don’t have the gun bones but still both in humanoid, it doesn’t work, i can just make it child of the hand but the mag is not moving. How i can handle that ? Just by creating some extra bones in blender and animating all my gun inside one rig by just changing the mesh or unity have a better way to add bones to an existing rig, and enable/disable inside unity ? I would like to do a gun switch of alot of weapon and of course use some layer.
Btw the 2 rig are extacly the same, except for the gun bones wich are not children of the humanoid bones.
Thanks for your answer.

Generally, you don’t have them in one mesh. Make the gun a separate mesh with it’s own separate animations, then parent it to the hand in Unity.

Yeah i tought about that, but in Blender if i don’t have the same rig it’s kinda hard to animate because i don’t have the hand and arm anymore, but i will try thank