Let's say I have a model with four arms, I want to add bones to the original _humanoid bone setup for those four arms.
Your talking custom skeleton here and currently there's no way to play as a model with a custom skeleton in Multiplayer.
But thats the least of your problems. You have a whole hell of a lot of animations to re-create to make this semi playable.
You can't just add bones the current humanoid animation file. You also can't add some four arms anims and then pick and use ones from the humanoid animation file.
Its all or nothing.
BUT, if you really WANT to make something like this. Take a look at the saber/assasin droids and rocket trooper. They use custom skeletons. They also only have a few rudimentary anims to make them work.