Why do you want to add charcoal to the feed?
You can do it, in grain sized pieces or dust is fine, but of course they will be ingesting very little or even no nutrients from the feed if you add charcoal to it. It absorbs everything, pretty much, not just toxins. It will also absorb the vitamins.
You can add feed to it to make them eat it. Mine used to eat the little bits they found after fires as well. You can add yoghurt to it to inspire them to eat it, or other stuff, but whatever food you add to it will likely be made null, so to speak, as far as nutrient value goes.
Are you sure it's from a good source, incidentally? Some people think any charcoal will do but you can't go using charcoal from treated wood or fires containing synthetic or chemical products without expecting harm to be done by it.
Best wishes.