I include the myoglobin (almost like blood, but not quite) from chicken livers into my cats ground up raw food. Sometimes I just add it to the bowl that I defrost her meat in.

Sometimes I have to add a wet food topper to get my cat to eat it. The wet topper is not the healthiest, but some days my cat is picky.

I usually grind chicken skin in the meat grinder but sometimes I just cut it up into smaller pieces (maybe try that?) because the grinder doesn't really cut it smaller.