Hi Madai,
I don't use any pork fat in my sausages, come to that I don't actually use any pork, but if you are looking for a substitute then check this thread
http://forum.sausagemaking.org/viewtopic.php?t=78&highlight=olive.
If you are trying to reduce the fat content however you would be better searching for postings by a member called Darkstream, who had a number of low fat recipes, arrived at by experimantation and help from other forum members.
I wouldn't go quite so far as Spuddy to say that nothing works like pork fat, and if you are willing to experiment and try out different options, I am sure that you will find something which you like and which suits your needs, dietary or otherwise.