The skin can be used in sausage but not as fat. The way I've used it is to first boil it for 60-90 minutes until it can be pierced with a knife, then after cooling, grind three times before adding to the sausage mince. Another way to use it is to make a rind emulsion by combining cooked skin, back fat and water in a bowl chopper. This video shows the resulting rind emulsion before it gets run through some type of high speed disperser. Note that they keep it very cold through this process.
A third way is to process the cooked skin with 10 % ice or stock in a bowl chopper (food processor for most of us) to form "skin-block", in German, schwartenblock. The skin is made of a protein called collagen which contains some gelatin. It gives the sausage a very "meaty" mouthfeel in my opinion and should help with water retention. It has very little nutritional value. It has a low cost, so it's use before labeling standards was probably much greater. As a starting point, I would suggest 5 % to a sausage to get a sense of it's effect.
Fashionably late will be stylishly hungry.