Basically, muscle (or protein, whatever you want to call it) taken from other bots needs to be broken down or converted into a form you can use. By that, I mean that if I eat a huge chunk of cow meat my own muscle doesn't grow right away. But it does give me the raw materials to make muscle when my body is ready.
That's why I was thinking that muscle <-> protein + nrg. Then protein is the stock and can be used to make shell, poison, muscle, or whatever.