I think I would start with more scientific solutions first.
1. Worm them. This is the very first thing you should do. Make certain they do not have lice or mites as well and cure that problem
2. Supplemental light to bring the total hours of light in the coop to 12-15
3. Cut back or eliminate the scratch feed so they will eat more of the complete chicken feed. Offer table scraps (especially meat) in place of the scratch feed
I promise if you do all of these it will help 1000% more than giving them pepper.
1. Worm them. This is the very first thing you should do. Make certain they do not have lice or mites as well and cure that problem
2. Supplemental light to bring the total hours of light in the coop to 12-15
3. Cut back or eliminate the scratch feed so they will eat more of the complete chicken feed. Offer table scraps (especially meat) in place of the scratch feed
I promise if you do all of these it will help 1000% more than giving them pepper.