Lots of people do

There's some advice to not feed raw mixed in with kibble (like you would a wet food) because they digest at different rates, but I've not seen this inforced strictly. I just wouldn't overdo it as you could risk unbalancing their diet, with it resulting, for example, in them getting too much protein, too much fibre, too much of certain vitamins (liver contains lots of vitamin A, which I imagine kibble will have as an artificial supplement, and it's possible to overdose in Vitamin A with some nasty side effects). The odd bit shouldn't affect it overall though.