You are very right to never add DE to Deep Litter as it will kill the necessary microbes.
I must warn you I am not a DE fan for a number of reasons.
DE needs to be dry to provide any benefit to insect/parasite removal (ie dust bathing). Putting it in their feed, into the gut, makes it wet, which prevents it from being effective. (Research has been ambiguous and sketchy on the effectiveness of DE in internal parasites).
Also, it is a well known fact that DE is very harmful to chicken (and human) lungs, another reason I wouldn't put it in their feed as it is more likely for them to inhale it. (I won't put it in my dust baths for that reason...I use wood ash intead with periodic permethryin).
If you want to help them eradicate worms (but it won't kill internal worms) "naturally," I recommend pumpkin seeds with strong garlic and paprika. Make up a mix that is crumbly, garlicy, and reddish in the food processor. That can help them expel worms, but it won't prevent the live worm from being picked up by the next bird. For true effectiveness with herbals, you need to rotate your field. If that isn't possible (small holding....ie backyard) inevitably worm growth is likely to happen and need treatment in older birds unless you are in a lucky location that has low worm load and heavy winters to kill them in the soil.
My thoughts and experiences.
LofMc
