3DS Max. Adding bones to a decompiled source model.

[The Robster]

I don't fully understand how skinning works when you import an smd but I need to either:
- Change weights on existing bones
- Add new bones to the model

Is there anything special I have to know about this? I've never been able to do anything with the skin modifier it automatically applies to imported models.
No, I don't plan on recompiling anything. I just need it to be usable inside 3DS Max.